Chapter 15: Renal Pathology

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Welcome back to The Deep Dive.

I have to be honest with you.

When I saw the reading list for today, I felt a little spike of cortisol.

Oh yeah.

Yeah, we are tackling a topic that I think makes a lot of med students and lifelong learners sweat.

We are looking at the kidney.

Specifically, we are doing a complete breakdown of chapter 15 renal pathology from the USMLE step one lecture notes.

It is a dense topic.

There is no denying that.

The kidney, you know, it has a reputation for being the most cerebral of the organ system.

Cerebral.

That's a good word for it.

Yeah, it's not just a pump like the heart or a bellows like the lungs.

It's a refinery.

And when you look at the pathology, it can feel like a list of abstract art descriptions.

Presence, trantrax, humps, spikes.

Exactly.

It sounds like a strange landscape rather than an organ.

It really does.

And I think for a lot of us, renal pathology feels like memorizing a phone book.

But our mission today is to demystify this.

We are going to treat this like a detective story.

I like that.

Because that's really what renal pathology is, isn't it?

You find a microscopic clue,

a cast in the urine or a pattern on a biopsy, and it unlocks the diagnosis for the whole patient.

That is the perfect way to frame it.

The kidney is a window into systemic health.

And while the terminology is heavy, the logic is actually quite beautiful once you see the structure.

We are going to follow the structure of the text exactly.

Okay.

We'll start with how the kidney forms or, you know, doesn't form congenital anomalies.

Then we will look at the cystic diseases.

And then we get to the main event, the part that gives everyone nightmares, the glomerulus.

We will break down the big divide between nephrotic and nephrotic syndromes.

That is the crucial high yield distinction.

I mean, if you get that right, everything else falls into place.

Then we will follow the plumbing down through the tubules, look at the blood vessels, check for stones, and finally discuss tumors and the bladder.

So we're going from the birth of the kidney all the way to the bladder exit.

It's a full site tour.

A full tour.

No stone left unturned, so to speak.

Very good.

Let's dive right in with section one, congenital anomalies.

This is where things can go wrong before the baby is even born.

And the first concept here is just the kidney not showing up at all.

Renal agenesis.

Right.

Agenesis.

Literally lack of creation.

This implies a failure during embryology where the ureteric bud fails to induce the blastema.

So they just miss their connection.

They just miss each other.

And this comes in two varieties.

And the difference is literally life and death.

You have unilateral agenesis where one kidney fails to form and bilateral agenesis where neither forms.

Let's start with unilateral.

If I'm born with only one kidney, am I in trouble?

Surprisingly, usually not.

If you have unilateral agenesis, the remaining kidney, the solitary one, undergoes compensatory hypertrophy.

It gets bigger.

It gets bigger.

It realizes it's flying solo.

It works harder.

And because of that redundancy built into the system, these patients often have adequate renal function.

They can live a normal life, often completely asymptomatic.

Wow.

So they might not even know.

They might not even know they only have one until they get an ultrasound for something else later in life.

That is amazing redundancy.

It speaks to how, I guess, over engineered the body is.

Yeah.

But bilateral agenesis, that's a different story.

Bilateral agenesis is incompatible with life.

And this brings us to a really important concept in renal pathology called the potter sequence.

Okay, let's unpack this potter sequence.

My intuition says no kidneys means the baby can't filter blood.

So they die of toxin buildup,

but that's not it.

That's not it at all because the fetus is hooked up to the mom.

Exactly.

The placenta is the ultimate dialysis machine.

It's doing the cleaning.

The problem isn't the chemistry.

It's the fluid mechanics.

Think about what amniotic fluid actually is.

By the second trimester, a huge component of amniotic fluid is fetal urine.

Right.

The baby is drinking it and peeing it out.

It's a closed loop.

The baby swallows the fluid, pees it out, and it cycles.

So no kidneys means no urine.

And no urine means oligohydramnios, too little amniotic fluid.

Now imagine the fetus in the uterus without that cushion of it.

It's just couscous squished.

The uterus contracts and compresses the fetus directly.

This physical compression is what causes the potter facies.

Potter facies, that's the description of the face, right?

Yes.

The text describes it as a flattened nose,

a recessed chin, and posteriorly rotated ears that are set low.

It's the result of being physically squished against the uterine wall.

And it affects the limbs, too.

It does.

You also see limb deformity, specifically something called talipase equinovirus.

Which is a fancy medical term for clubfoot.

Correct.

Talipase involves the ankle and foot, equino means the heel, varus means turned inward.

But the tragedy of the potter sequence isn't just the face or the feet.

You can survive a clubfoot.

The most lethal consequence of having no amniotic fluid is pulmonary hypoplasia.

The lungs don't develop.

Right.

It turns out for lungs to develop properly in utero, they need to breathe

The hydraulic pressure of the amniotic fluid expands the airways and stimulates growth.

Oh, I never knew that.

Yeah.

Without that fluid, the chest is compressed, the lungs remain tiny and underdeveloped.

So even if you could somehow fix the kidney issue, these infants have underdeveloped lungs, which is why it is incompatible with life.

It's a domino effect.

No kidney leads to no fluid, which leads to compression and bad lungs.

It's a tragic sequence.

Precisely.

It all starts with the lack of urine production.

Now, moving away from missing kidneys, let's talk about misshaped ones.

The text mentions

hypoplasia.

Hypoplasia is distinct from a genesis.

The kidney is there, but it failed to develop to a normal weight.

It's small.

It's underdeveloped.

Underdeveloped.

It usually has a decreased number of calluses and lobes.

It's usually unilateral.

And like a genesis, if the other kidney is good, the patient might be fine.

Then we have the horseshoe kidney.

I love this visual because it's exactly what it sounds like.

It is a very common anomaly.

Something like one in 600 abdominal x -rays will show this.

The kidneys are fused together, usually at the lower pole.

So you get this big U shape.

A continuous U shape or horseshoe shape spanning across the midline instead of two separate bean shapes.

Does it work?

Is the function okay?

It works fine.

The tissue itself is normal.

Renal function is usually preserved.

But because of the abnormal anatomy, there is a catch.

Normally, kidneys ascend from the pelvis to the upper back during development.

A horseshoe kidney tries to ascend, but it gets hooked.

It gets stuck on a blood vessel.

Correct.

The text specifies it gets caught on the inferior mesenteric artery, so it sits lower in the abdomen than normal.

And does that position cause problems?

It predisposes the patient to renal calculi kidney stones.

The drainage isn't quite as efficient and stasis, you know, fluid just sitting there leads to stones.

So functional tissue, but bad plumbing.

Bad plumbing.

That seems to be a theme.

And finally, for congenital stuff, we have the ectopic kidney.

Ectopic just means out of place.

The most common abnormal location is a pelvic kidney.

It just didn't ascend at all.

So it's just sitting down in the pelvis.

Does the kidney and the pelvis work?

Usually, yes.

But again, the plumbing is the issue.

The ureters, the tubes draining the urine, can be torturous or twisted because they are taking a weird path to the bladder.

And a twisted hose doesn't drain well.

Exactly.

That tortuosity predisposes the patient to pylonephritis, which is a kidney infection.

So the theme with these structural anomalies, horseshoe and ectopic, is that the kidney tissue works, but the weird plumbing leads to stones or infections.

That is a fair summary.

Absolutely.

Alright, let's move to section two.

Cystic diseases.

This is where the kidney architecture is replaced by fluid -filled sacs.

It looks like a sponge.

The text breaks this down into two main types of polycystic kidney disease, or PKD.

And the distinction here is genetic.

Correct.

You have the autosomal recessive form and the autosomal dominant form.

And the clinical pictures are vastly different.

It's really important to keep them separate.

Let's start with the one that affects babies,

autosomal recessive.

This is also called infantile polycystic kidney disease.

As the name suggests, it presents an infancy.

It is rare, but it is severe.

It involves a mutation in the PKHD1 gene.

What does the kidney look like?

I'm imagining it's just full of these cysts.

They are bilaterally enlarged.

Massive.

The text describes the cut surface as sponge -like because of the thousands of tiny cysts.

And since it's happening in infancy, do we see that fluid issue again, the plotter sequence?

We do, because renal failure happens so early, you can see pulmonary hypoplasia similar to what we discussed.

But there is another key association here that you have to know.

The liver.

The liver.

What does the liver have to do with it?

Patients with the recessive form often have congenital hepatic fibrosis.

The text mentions hepatic cysts and the development of liver cirrhosis even in childhood, so it's a hepatorial issue.

So recessive is the rapid infantile form with liver problems.

Now contrast that with the autosomal dominant form.

This is the one we see in adults.

Right.

This is much more common about one in 1 ,000 people.

It's an autosomal dominant mutation.

The text points out two genes,

PKD1 on chromosome 16, which codes for polycystin 1 and PPC2.

I've heard a mnemonic for this.

Polycystic kidney has 16 letters, so it's chromosome 16.

That is a classic medical school hook.

Yes, it works.

Now the fascinating thing here is the timeline.

We call it adult PKD because patients are totally asymptomatic until middle age.

Why is that?

If you have the gene from birth, why does it take 40 years to show up?

It's the clinical lag.

The text explains this really well.

The cysts are there from birth, but they initially involve less than 10 % of the nephrons.

So you've got 90 % of your kidneys still working fine.

Exactly.

You have plenty of reserve.

But over decades, those cysts slowly, insidiously expand.

As they get bigger and bigger, they physically compress the healthy tissue around them.

Eventually, you run out of reserve.

So you hit 40 or 50, and suddenly you have symptoms.

What are they?

Renal insufficiency is the end result.

But before that, hypertension is a big one.

Haematuria, which is blood in the urine, often happens when a cyst bursts.

Flank pain.

Sometimes they can feel an abdominal mass because these kidneys get massive.

The text describes massive bilateral enlargement.

Yes.

We are talking about kidneys that can be the size of footballs, filled with cysts containing serous fluid, turbid fluid, or even hemorrhagic fluid if one bleeds into itself.

And just like the recessive form had the liver connection, this one has extra renal danger zones too.

It does.

And this is high yield.

We see cysts in the liver and pancreas, but the scary association is the Barry aneurysm.

In the brain.

That's a ticking time bomb.

In the circle of Willis, yeah.

If that ruptures, it's a subarachnoid hemorrhage.

The worst headache of my life scenario.

So you have a patient with kidney cysts, but you need to be worried about their brain vascularity.

Also, mitral valve prolapse and colonic diverticula are associated.

So it's almost like a systemic connective tissue weakness in a way.

In a sense, yes.

The polycystin protein is involved in cell to cell adhesion and cilia function, so when it's faulty, things start to balloon out.

Before we leave cysts, let's quickly hit the other ones mentioned.

Renal dysplasia and acquired polycystic disease.

Renal dysplasia is important because it's the most common cystic disease in children.

It's different from PKD because it's usually unilateral and not inherited.

Okay, so what's the key feature?

The key feature here, the buzzword, is the presence of cartilage in the renal parenchyma.

Cartilage in the kidney.

That definitely doesn't belong there.

No.

It's a sign that the tissue development, the differentiation went haywire.

Okay.

And acquired polycystic disease.

That's seen in patients who are already on dialysis for other reasons.

Their shrunken, failing kidneys start to develop cysts.

The concern there is a risk of renal cell carcinoma developing in those cysts.

And finally, medullary cysts.

Two types to know.

Medullary sponge kidney, usually innocuous.

Maybe some stones, but generally not a big deal.

And then nephanophysis.

That is a mouthful.

It is.

Nephanophysis medullary cystic disease complex.

This one is not innocuous.

It causes progressive renal failure in young adults or children.

The cysts are located specifically at the corticomedullary junction, and the kidneys are shrunken, not enlarged, like in PKD.

Okay.

We've built the kidney.

We've looked at cysts.

Now we're going to zoom in.

We are going down to the microscopic level, to the glomerulus.

This is section three.

The glomerular framework.

This is the heart of renal pathology.

The glomerulus is the filter.

And when it gets damaged, it breaks in one of two distinct ways.

You have to understand this binary.

Nephrinic versus nephrotic.

The text has a table for this.

It's like a compass.

Let's calibrate.

Nephritic syndrome.

Think I for inflammation.

Nephritic is an inflammatory process.

The filter is inflamed and angry.

This leads to hematuria blood in the urine.

And not just any blood.

You're looking for something specific.

Specifically, we look for RBC casts.

That proves the blood is coming from the kidney tubules, not the bladder.

It's a key diagnostic clue.

And because it's inflamed, the filter clogs up.

Yes.

It gets congested.

You get oliguria low urine output.

You get azotemia rising waste products like BUN and creatinine in the blood.

And you get hypertension.

What about protein?

There is some protein area, but it's mild.

Usually less than 3 .5 grams per day.

The inflammation creates some holes, but not giant ones.

So nephritic equals inflammation, blood, hypertension.

Now contrast that with nephrotic syndrome.

Nephrotic is the leaky profile.

It's not so much inflamed as it is structurally damaged in a way that lets protein pour out.

The hallmark is massive proteinuria.

More than 3 .5 grams per day.

So the floodgates are open for protein.

And when you lose all that protein, what happens to the patient?

Hypoalbuminemia.

You lose albumin in the urine, so levels in the blood drop.

Albumin acts like a sponge to hold fluid in your vessels.

Without it, fluid leaks into the tissues.

And that causes?

Generalized edema.

They are puffy.

Puffy face, swollen legs, fluid in the abdomen.

And strangely,

high lipids.

Why does that happen?

Yes, hyperlipidemia and lipiduria.

The liver notices the low protein in the blood and tries to compensate by churning out more albumin.

But it's a blunt instrument.

It also churns out lipids at the same time.

So nephrotic is massive protein, edema, and lipids.

Correct.

And to diagnose which specific disease is causing either of these syndromes, we need a renal biopsy.

And we look at it three ways.

The holy trinity of renal pathology.

Light microscopy.

L .M.

What does the tissue look like under a standard scope?

Immunofluorescence.

I .F.

We use glowing antibodies to see if there are immunoposites.

And electron microscopy.

E .M.

What's happening at the nano level?

Specifically with the basement membrane and podocytes.

OK.

Let's apply this.

Section 4.

Primary glomerulopathies presenting as nephrotic syndrome.

The blood and inflammation group.

First up is a classic.

Acute post -treptococcal glomerulonephritis.

A .P .S .G .N.

This is the classic story.

A young child, maybe six or seven, they had a sore throat or a skin infection like impetigo about two to four weeks ago.

The text specifies group of beta -hemolytic strep.

Right.

The infection clears but then weeks later the child presents with cola -colored urine.

Smoky urine.

That's a very specific description.

It's because the blood has had time to get oxidized in the acidic urine, right?

Exactly.

It's not bright red.

They have periorbital edema, so puffy eyes, and hypertension.

It's an immune complex disease.

The body made antibodies against the strep and those complexes got stuck in the kidney glomeruli.

What do we see on the biopsy?

The holy trinity.

On light microscopy, the glomerulus is packed with neutrophils.

It's hypercellular.

It's clearly inflamed.

Okay, so LM shows inflammation.

What about IF?

Immunofluorescence shows a granular pattern of IgG in C3.

It's often described as lumpy bumpy.

The immune complexes are just dropped there randomly.

Lumpy bumpy.

And the EM clue.

The electron microscopy.

Sub -epithelial humps.

These are the immune complexes sitting under the epithelial cells, the podocytes.

They look like little camel humps on the outside of the basement membrane.

Well,

maybe just humps equals post -trip.

What's the prognosis?

In children, excellent.

Conservative management, fluid control, 95 % recover completely.

Adults have a bit of a rougher course, but kids usually bounce back just fine.

Next up is the scariest sounding one.

Rapidly progressive glomerulonephritis, or RPGN.

RPGN isn't a single disease.

It's a clinical picture of rapid deterioration of renal function.

The hallmark structure here is the crescent.

We see crescents in Bowman space.

What is a crescent made of?

This is a classic question.

The crescent is composed of fibrin and macrophages, plus some parietal cells.

It's a sign of severe damage.

The glomerular capillaries have ruptured, fibrin has leaked out, and macrophages have rushed in to clean up.

The crescent crushes the glomerulus.

It physically squeezes it to death.

Essentially, yes.

The text breaks RPGN down into three types based on the immunofluorescence pattern.

Type 1 is linear.

Linear IF means anti -GBM disease.

The antibodies are attacking the glomerular basement membrane directly, laying down in a smooth, continuous line.

This is good pasture syndrome.

Good pasture.

That affects the lungs, too.

Yes.

The same type IV collagen in the GBM is in the basement membranes in the lung alveoli.

So the antibody attacks both.

You get hemoptysis, so coughing up blood, A &E, renal failure.

It typically affects young males.

Type 2 RPGN.

That's immune complex mediated.

It's essentially a complication of other diseases.

The immune complexes from post strep or lupus get so bad they cause crescents, the IF will be granular or lumpy bumpy, just like we saw before.

So it's like APSGN on steroids.

In a way, yes.

A much more severe presentation.

And type 3.

Type 3 is pouchy immune.

Pouchy meaning a distinct lack of.

The IF is negative.

There are no deposits.

But these patients are positive for ANCA in their blood.

This is usually a manifestation of a systemic vasculitis.

ANCA positive.

OK.

So crescents equals RPGN.

Then you check the IF to see which of the three types it is.

Exactly.

Linear, granular, or negative.

Moving on.

The most common glomerulonephritis in the world, IgA nephropathy.

Also called Berger disease.

Not Berger's disease.

The smoker's vasculitis.

Right.

Berger.

B -E -R -G -E.

Correct.

This affects children and young adults.

Mostly males.

The classic presentation is recurrent gross hematuria that happens right after or even during a respiratory infection.

Post strep was two, four weeks after.

This is immediate.

Correct.

That's a key distinction.

IgA happens concurrently with the cold.

The patient has a sore throat.

And a day or two later, they see blood in their urine.

Post strep has that latent period.

What's happening in IgA nephropathy on a microscopic level?

We see IgA deposits in the mesangium.

Which is the structural support of the glomerulus.

It's often associated with celiac sprue or Hanock -Schenlein purpura, HSP.

HSP is that systemic childhood disorder, right?

Right.

Pupura on the buttocks and legs.

Palpable purpura on legs and buttocks.

Abdominal pain, joint pain.

HSP is basically systemic IgA nephropathy.

The IgA deposits are everywhere, not just the kidney.

Okay.

Next is MPGN.

Membrano proliferative glomerulonephritis.

That is a long name.

It is, but it describes the appearance.

Membrano refers to the basement membrane thickening.

And proliferative refers to the mesangial cells proliferating.

So you get thick membranes and more cells.

The visual clue here is tram tracking.

The mesangial cells grew out and interposed themselves into the basement membrane, causing it to split.

On a silver stain, it looks like two parallel lines tram tracks.

The text mentions type I and type II.

What's the difference?

Type I is associated with hepatitis B and C.

It has some endothelial immune deposits.

Type II is also called dense deposit disease.

It's associated with something called C3 nephritic factor.

C3 nephritic factor.

What does that do?

It's an autoantibody that stabilizes C3 convertase, which is an enzyme in the complement cascade.

So you chew up all your complement.

These patients have very low serum C3 levels.

Okay, last one for the nephritic side, Alport syndrome.

Alport is genetic.

It's an X -linked defect in type IV collagen, specifically the COL4A5 gene.

Type IV collagen is the basement membrane stuff.

The filter itself is just built wrong.

Right.

The basement membrane is weak and defective from the start.

The triad here is crucial.

Hereditary nephritis plus deafness plus eye disorders like cataracts.

Can't see, can't pee, can't hear a bee.

That's the mnemonic that gets everyone through the exam.

The visual on electron microscopy is a basket weave appearance.

The basement membrane is splitting and relaminating because the collagen is defective.

It looks all woven and irregular.

All right, that was a workout.

That was the nephritic side, the inflamed bloody side.

Now let's switch gears to section five, nephrotic syndrome.

The leaky side, massive protein loss.

First up, membranous glomerulonephritis.

This is a common cause in adults, particularly Caucasians.

What's the mechanism?

Immune complexes, again.

But here, they form subpithelially on the outer side of the basement membrane.

The text notes that most cases are idiopathic, involving autoantibodies against podocyte antigens like the PLA2 receptor.

And the visual.

What do we see on biopsy?

Diffuse thickening of the cacillary walls on light microscopy.

But the key is the silver stain.

It shows spikes.

Spikes.

So the basement membrane grows up around the deposits, creating these little spiky projections.

Exactly.

It's called the spike and dome pattern on EM.

The spikes are the new basement membrane and the domes are the immune deposits.

Are there any important associations with membranous?

Absolutely.

It's associated with solid tumor's lung and colon cancer and infections like hep B and C or lupus.

If you see membranous in an older adult, you have to look for an underlying cancer.

Next is the one for the kids.

Minimal change disease.

This is the most common cause of nephrotic syndrome in children.

And the name minimal change comes from the light microscopy.

It looks normal.

It looks completely normal.

You look at the biopsy and the glomerulus looks fine, but the kid is swollen like a balloon with massive pertinuria.

So you have to dig deeper.

Right.

You have to go to electron microscopy to see the problem.

On EM, you see effacement of podocyte foot processes.

The little feet that wrap around the capillaries have fused and flattened out.

They've been erased, basically.

Yes, effaced means erased.

And this causes the leakiness.

And there's lipid in the tubules.

Yes, hence the old name lipoid nephrosis.

You see lipids that have been reabsorbed by the proximal tubular cells.

The good news here.

Excellent prognosis.

It responds dramatically to corticosteroids.

It's one of the few renal diseases we can often fix with a simple pill.

But if it doesn't respond to steroids,

maybe it's the next one.

FSGS, focal segmental glomerulus sclerosis.

This is the bad actor.

It's the most common cause of nephrotic syndrome in adults in the US, especially in African Americans and Hispanics.

OK, let's break down the name.

Focal segmental glomerulus sclerosis.

Focal means only some of the glomeruli are affected.

Segmental means only a part of each affected glomerulus is scarred.

And glomerulus sclerosis just means scarring of the glomerulus.

The chlorosis means scarring.

Got it.

Yes.

And unlike minimal change disease, this does not respond well to steroids.

It often progresses to chronic renal failure.

What are the associations?

What causes it?

HIV is a big one.

There's a collapsing variant called HIV -associated nephropathy.

Also, heroin use, sickle cell anemia, and massive obesity.

It's a disease of stress and toxins on the podocytes.

So, to recap the nephrodotes, kids with a normal -looking biopsy that responds to steroids is minimal change.

Adults with spikes on silver stain, you think membranous.

And sclerosis, scarring, especially with HIV or heroin, that's FSGS.

That's a great shorthand.

Perfect.

Section 6 touches on secondary cholera disease.

Diabetes mellitus is huge here, right?

It's a massive cause of kidney failure.

Diabetes is the leading cause of end -stage renal disease in the developed world.

Pathologically, we look for Kimmel -Steel -Wilson nodules.

Kimmel -Steel -Wilson.

These are nodular glomerulosclerosis, big pink high -line balls in the glomerulus.

The mechanism involves non -enzymatic glycosylation of the basement membrane, and, very importantly, high -line arterial sclerosis of the efferent arteriole, which increases pressure inside the glomerulus.

And section 7 takes us out of the glomerulus and into the tubules and interstitium.

So, tubulo -interstitial nephritis.

This includes infections and drug reactions.

Let's talk about acute pylonephritis.

This is a kidney infection, usually caused by bacteria climbing up the ureter.

It's an ascending infection, yes.

Usually E.

coli coming from the GI tract.

It starts as a bladder infection or a cystitis and goes upstream to the kidneys.

How do we tell the difference clinically between a simple bladder infection and pylonephritis?

Because they both can cause pain with urination.

Pylonephritis is systemic.

You have fever, chills, and the classic sign.

Costo -verbal angle tenderness.

CVA tenderness.

A flank pain.

Yeah, if you tap on their back over the kidney, they jump.

That's a huge clue.

It's in the kidney, not just the bladder.

And the urine finding.

There's a key one.

WBC casts.

This is critical.

If you see white blood cells in urine, it could be cystitis.

If you see WBC casts, these are cylindrical clumps of WBCs molded in the shape of the tubule.

That means they came from the tubules.

That proves it's in the kidney.

So casts mean kidney.

Casts mean kidney.

What about chronic pylonephritis?

That's usually due to chronic obstruction or reflux, which is urine flowing backward up the ureter.

The scarring creates a very specific look.

The tubules get filled with colloid -like material and look like thyroid tissue.

We call it thyroidization of the kidney.

That's a cool term.

Now, what if the inflammation isn't from bacteria but from a drug, drug -induced interstitial nephritis?

This is a hypersensitivity reaction.

A type IV hypersensitivity.

The common culprits are penicillins, NSAIDs, and diuretics.

It happens about two weeks after starting the drug.

Symptoms.

The triad is fever, rash, and eosinophilia.

High osinophils in the blood or urine is the giveaway.

If you see osinophils and acute renal failure, you have to think about a drug reaction.

And it's reversible.

Usually yes if you stop the offending drug.

Section 8.

Acute tubular injury, or ATI.

The text says this is the most common cause of acute renal failure.

It is.

This is when the tubular cells themselves die and slow off into the lumen.

And when they slough off, they form the classic muddy brown casts.

Also called dirty brown granular casts.

If you hear muddy brown casts, the answer is almost certainly acute tubular necrosis or injury.

What causes the cells to die like that?

Two main categories.

Ischemic, so things like shock, hemorrhage, severe dehydration where the kidney doesn't get enough blood,

or nephrotoxic.

Cocsins.

Drugs like aminoglycosides, gentamicin is the classic example.

Radiographic contrasts die.

Heavy metals like lead.

And a classic board question topic.

Ethylene glycol antifreeze.

Antifreeze, that's a classic board question.

Why does antifreeze hurt the kidney?

It gets metabolized to oxalic acid, which then precipitates as calcium oxalate crystals in the tubules.

It physically shreds the cells.

Is ATI reversible?

It is potentially reversible.

The tubular cells can regenerate.

They have a good blood supply.

But the patient needs support, maybe dialysis, while the tubules regrow.

It has phases.

An initiation phase, a maintenance phase where they don't pee, and a recovery phase where they start to pee a lot.

Moving to section nine.

Urolithiasis.

Kidney stones.

Everyone's nightmare.

Extremely painful but very high yield.

We characterize stones by their composition.

Most common type.

Calcium oxalate.

About 75%.

These are radiopic.

You can see them on a plain KUB x -ray.

And the text notes a connection to urine pH.

Yes, calcium oxalate stones tend to form an alkaline urine.

They are also linked to ethylene glycol, as we just said, or even vitamin C abuse.

What about the staghorn calculi, the big monster stones that fill the whole collecting system?

Those are struvite stones, which are magnesium ammonium phosphate.

They are caused by infection with urea -splitting bacteria, like Proteus or Eclipsiella.

So the bacteria are actually causing the stone?

Yes.

The bacteria turn urea into ammonia, making the urine very alkaline, which precipitates the stone.

It fills the whole renal pelvis and calluses like a cast of the staghorns.

And uric acid stones?

Seen in patients with gout or during chemotherapy for leukemia when there's high cell turnover.

These form in acidic urine.

And importantly, they are radiolucent.

They don't show up well on regular x -rays.

You need a CT scan.

Okay.

Section 10.

Vascular disorders.

The blood supply.

Renal artery stenosis.

This is narrowing of the renal artery.

The kidney thinks the body's blood pressure is low because it's not getting enough flow, so it churns out renin.

And the renin angiotensin aldosterone system kicks in and raises systemic blood pressure.

Right.

So you have a patient with hypertension that is very resistant to medications.

Who gets this?

Two main groups.

Old men with atherosclerosis, so plaques in the renal artery, and young women with fibromuscular dysplasia.

That's the string of beads appearance on angiography.

Yes.

The artery wall thickens in segments, looking like beads on a string.

It's a classic visual.

And briefly, nephrosclerosis.

The nine nephrosclerosis is the fine granular letter grain appearance you see on the surface of kidneys from long -standing hypertension.

Malignant nephrosclerosis is from accelerated severe hypertension.

And that looks different.

Yes.

That gives you the phlebitin appearance pinpoint patechial hemorrhages on the kidney surface due to fibrinoid necrosis of the small vessels.

Section 11.

Tumors.

Let's talk about the bad mass.

Renal cell carcinoma, RCC.

This is the most common kidney cancer in adults, typically an older male who smokes.

There is a strong genetic link to von Hippel and Dau or VHL disease, especially for the clear cell type.

What's the classic presentation?

The classic triad is hematuria, a palpable mass, and flank pain.

But the text warns only about 10 % of patients have all three.

Painless hematuria is often the most common single sign.

RCC is also famous for perineoplastic syndromes.

It produces hormones it shouldn't.

Oh, yes.

It can make EPO erythropoietin causing polycythemia, so too many red blood cells.

It can make renin causing hypertension.

It can make a PTH -like peptide causing hypercalcemia.

It's a very metabolically active tumor.

It's a rogue hormone factory.

Where does it spread?

Uniquely, it loves to invade the renal vein.

It can grow as a solid cord of tumor right up the vein into the inferior vanicaba and even into the right side of the heart.

That is terrifying and microscopic types.

Clear cell is the most common.

It's linked to VHL and loss of chromosome 3P.

The cells look clear because they are full of lipids and glycogen.

Then you have papillary, which is often bilateral and linked to the met protokinogen.

And chromophobe, which is more rare and has darker cells.

For kids, the big kidney tumor is Wilms' tumor.

Also called nephroblastoma.

You'll see a huge abdominal mass in a child, usually between age two and five.

The associations are alphabet soup, Weigree syndrome.

Weigree stands for Wilms' tumor, aniridia, which is no iris in the eye, genital anomalies and retardation, mental and motor.

This is linked to a WT1 gene deletion.

And Beckwith -Wiedemann.

That's the overgrowth syndrome.

A big tongue, big body, hemihypertrophy, where one side of the body is bigger than the other and a risk for Wilms' tumor.

This one is linked to the WT2 gene.

Finally, let's finish the plumbing.

Section 12, the lower urinary tract,

the bladder.

Most common pathology here is cystitis inflammation of the bladder.

Women get it more than men because of a shorter urethra, making it easier for bacteria to ascend.

Usually E.

coli.

But what about hemorrhagic cystitis?

Blood in the bladder?

That's often a side effect of cyclophosphamide, which is a chemotherapy drug or certain viruses like adenovirus.

And bladder cancer.

Transitional cell carcinoma, or TCC, is the big one in the US and Europe.

The classic sign is painless hematuria.

The risk factors are P -related.

What do you need by that?

Carcinogens that are excreted in the urine sit in the bladder for hours.

So smoking is a huge risk factor, and so are azo dyes used in the textile industry.

The carcinogens are just sitting there bathing the bladder wall.

But there is a specific type called squamous cell carcinoma linked to a parasite.

Yes.

This is a huge deal in other parts of the world.

Cystosoma hematobium.

It's a fluke found in Egypt and the Middle East.

The parasite eggs lodged in the bladder wall cause chronic inflammation,

and that inflammation eventually leads to squamous cell carcinoma.

That is a specific fascinating connection.

It is.

It's a classic example of chronic inflammation leading to cancer.

We have covered a massive amount of ground, from the potter sequence in the womb through the complex world of the glomerulus.

All those humps, spikes, and crescents.

Right.

Pass the stones and down to the bladder tumors.

It is a long journey, but the logic holds up.

Inflammation leads to blood, which is nephrotic.

Structural damage leads to protein loss, which is nephrotic.

Obstruction leads to infection and stones.

So what does this all mean for us?

What's the big picture takeaway?

I think the provocative thought here is about redundancy and compensation.

Think about it.

You can be born with one kidney and never know it.

Your body just adapts.

You can have autosomal dominant polycystic kidney disease and lose 90 % of your nephrons before you even feel a symptom.

The remaining 10 % work over time.

The body fights to stay normal.

It does.

It compensates until it absolutely can't anymore.

It highlights the incredible resilience of the renal system, but also the danger.

Because by the time you have symptoms, the damage is often extensive and irreversible.

A sobering but appreciative thought.

Thank you for walking us through the microscopic maze of the kidney.

It makes a lot more sense now.

My pleasure.

It was fun.

And to you, the listener, thanks for studying with the Last Minute Lecture team.

We'll see you on the next deep dive.