01Musculoskeletal
RA vs OA: joint involvement patterns
Why RA spares the DIP and OA hits it. It's not random.
Concepts
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Walk through a concept. One tap at a time.
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Walk 01 · 10 stepsCoagulative necrosisGhost cells, firm infarcts, and why the brain is different.Start walking →
Walk 02 · 10 stepsLiquefactive necrosisWhen enzymes win: pus, abscesses and brain infarcts.Start walking →
Walk 03 · 10 stepsApoptosisThe quiet death: BCL2, cytochrome c and caspases.Start walking →
Walk 04 · 10 stepsLeukocyte extravasationMargination to chemotaxis: how neutrophils leave the blood.Start walking →
Walk 05 · 10 stepsThrombosis and Virchow's triadThree factors, lines of Zahn and the fate of a thrombus.Start walking →
Walk 06 · 10 stepsGranuloma formationTh1, IFN gamma, epithelioid cells and caseation.Start walking →
Quick reads
02Blood vessels
Aneurysm
A weakened vessel wall giving way to pressure. Every cause is just a different way of weakening that wall.
03General pathology
Transudate vs exudate
People mug up Light's criteria, then go blank in the exam. Ask: what's actually leaking out?
04Hematology
BT, PT, INR, aPTT, TT
Five tests that all sound like they measure "clotting". Each one checks a different part of the system.
05Blood bank
Direct vs indirect Coombs
Same reagent in both. The only difference is where the antibody is sitting.
06Immunology
Low C3 + low C4 vs low C3 + normal C4
Stop memorising disease lists. Ask one question: was C4 used?
RA vs OA: joint involvement patterns
Students memorise "RA spares DIP, OA hits DIP" as random facts, then blank on which is which.
It's not random. Just ask: what tissue is the disease attacking?
RA attacks synovium. It's immune driven - a pannus grows out of synovial lining and eats the joint. So RA goes where synovium is in great density: the MCP (metacarpophalangeal/knuckle), PIP (proximal interphalangeal) and wrist. The DIP (distal interphalangeal) has little synovium, so it's spared. And since the trigger is systemic (circulating antibodies, cytokines), it hits both hands symmetrically.
Proof of the rule - in the spine, RA ignores everything except the atlantoaxial joint. Why? Discs aren't synovial; the atlantoaxial joint is! RA literally hunts synovium across the skeleton.
OA is different. It's cartilage breaking down under mechanical load - wear and tear, not immunity. So it follows wherever you grind/hardworking joint: the DIP (fine pinch), thumb base, knees, hips, spine. Being mechanical and local, it's often asymmetric - your dominant hand, your old injured knee.
Now the part students miss - which joints OA spare, and why. OA almost never touches the MCP, wrist, elbow or ankle. Look at the logic: these aren't built to take pure compressive load the way fingertips, knees and hips are. The ankle especially has tough, resilient cartilage and great congruence, so it resists wear beautifully. So if you see OA at the MCP, wrist or ankle, don't accept it at first sight - think secondary cause: haemochromatosis, CPPD (calcium pyrophosphate dihydrate deposition - pseudogout), old trauma. (Remember ''-1 mark'' is for wrong click)
Blood vesselsAneurysm
A weakened vessel wall giving way to pressure. Think of the artery as a pipe under constant pressure. As long as the wall is strong, it holds its shape. An aneurysm begins the moment the wall loses its strength in one spot - and because blood pressure is always pushing outward, that weak spot balloons out.
That’s the whole definition: a permanent, localised dilatation of a vessel, born from a wall that can no longer resist the pressure inside it. So every cause of aneurysm is just a different way of weakening that wall.
- Atherosclerosis weakens it by damaging and thinning the intima and media (the commonest cause, classically the abdominal aorta).
- Chronic hypertension strains it.
- Cystic medial degeneration weakens the elastic tissue in the young - the Marfan/connective-tissue story.
- Infections weaken a focal patch (mycotic aneurysm; syphilis classically hitting the ascending aorta).
- Trauma tears it.
Different aetiologies, one shared endpoint - a wall too weak for the pressure it holds.
Now the types, which fall straight out of how the wall fails. If all three layers of the vessel bulge out together but stay intact, it’s a true aneurysm - the wall is stretched but continuous (the typical atherosclerotic aortic aneurysm, either fusiform, ballooning symmetrically all around, or saccular, a one-sided outpouching).
If the wall is actually breached and blood escapes but is contained by surrounding tissue and clot, it’s a false (pseudo) aneurysm - not a true dilatation but a walled-off leak.
And there’s a crucial cousin that isn’t a true aneurysm at all: dissection, where blood tears into the wall itself, splitting the layers apart and tracking along them - driven by hypertension and cystic medial degeneration.
The pathophysiology under all of this is the same vicious cycle: a weak wall dilates, and by the law of Laplace, a wider vessel experiences more wall tension at the same pressure - so dilatation begets more tension, which begets more dilatation. The aneurysm feeds its own growth, which is exactly why they tend to enlarge over time rather than stay stable.
That self-enlarging nature is what makes the symptoms and complications a matter of when, not if. For most of its life an aneurysm is silent - it grows quietly, felt as nothing, sometimes discovered by chance as a pulsatile abdominal mass.
Symptoms appear only when it gets big enough to press on neighbours (pain, or a thoracic aneurysm compressing the oesophagus or airway) or when it starts to threaten failure. And the complications all trace back to that same weak, stretched wall.
Rupture is the catastrophe - the wall finally gives, causing massive internal bleeding (a ruptured AAA presents as sudden severe abdominal/back pain, a pulsatile mass, and shock - a surgical emergency with high mortality).
Along the way, the sluggish, turbulent blood inside the dilated sac tends to clot, so thrombosis and embolism are common - a clot forms in the aneurysm and throws off fragments downstream.
General pathologyTransudate vs exudate
People mug up Light's criteria, then go completely blank on it in the exam.
But there's actually a logic here. Get it once and you'll never mix them up again.
Just ask yourself one thing - what's actually leaking out of the vessel?
💧 Transudate = pressure problem. The vessel wall is totally fine. Fluid is just getting pushed out (high hydrostatic pressure, like in heart failure) or not held back properly (low oncotic pressure, like cirrhosis or nephrotic syndrome). Wall's intact, so only water and small stuff gets through. That's why it's thin. Low protein, low LDH, clear, hardly any cells.
💭 Exudate = wall problem. Inflammation has chewed up the vessels and made them leaky. Now the big stuff (protein, LDH, actual cells) comes pouring out through gaps that should not be there.
That's why it's rich. High protein, high LDH, cloudy, full of cells. Infection, malignancy, TB, PE, pancreatitis, RA - all of them are damaging or inflaming the wall.
And honestly that's all Light's criteria is doing. Every single cutoff is just asking the same question - did protein and LDH leak out or not? If yes, the wall got breached. Exudate.
HematologyBT, PT, INR, aPTT, TT
Five tests that all sound like they measure "clotting," and students memorise their normal values without knowing what each one is actually looking at. But each test checks a different part of the system, and once you know which part, the conditions sort themselves.
Start with the big split. Clotting has two arms - the platelet side (forming the initial plug) and the coagulation factor side (the cascade that lays down fibrin to cement it). Different tests probe different arms.
1️⃣ BT (bleeding time): checks the platelet plug - the very first response to a cut. So BT is about platelets: their number and their function. It's prolonged when platelets are low (thrombocytopenia) or not working (von Willebrand disease, aspirin, uremia). If BT is up, think platelet problem, not coagulation factor problem.
Now the cascade. The classic teaching splits it into two pathways that meet at a common point - and the two main tests each cover one pathway.
2️⃣ PT (prothrombin time): measures the extrinsic pathway (factor VII) plus the common pathway. The memory hook is PT = Play Tennis = outdoor game - extrinsic factors. Crucially, PT tracks the extrinsic arm - which is why it's the one sensitive to warfarin and to liver disease and vitamin K deficiency (factor VII drops first).
2️⃣🆕️ INR: is just PT standardised. Different labs use different reagents, so PT values weren't comparable between labs. INR is a maths correction that makes PT mean the same thing everywhere - that's why warfarin is monitored by INR, not raw PT. Same test, universal scale.
3️⃣ aPTT (activated partial thromboplastin time): measures the intrinsic pathway (factors VIII, IX, XI, XII) plus the common pathway. Memory hook: aPTT = Table Tennis (or a = andar) indoor game - intrinsic. This is the one prolonged in haemophilia (VIII and IX) and the one used to monitor heparin.
4️⃣ TT (thrombin time) jumps to the very last step: fibrinogen turning into fibrin. So TT bypasses the whole cascade and tests only the final conversion. It's prolonged when fibrinogen is low or abnormal, or when heparin is around.
So now read any condition by which test moves:
- Haemophilia A/B → aPTT up; PT normal (intrinsic problem only)
- Warfarin / vitamin K deficiency / liver disease → PT/INR up first (extrinsic VII is most sensitive).
- Heparin → aPTT up (and TT up too).
- von Willebrand / platelet problems / aspirin → BT up.
- DIC → everything up - BT, PT, aPTT, TT - because you've consumed it all (ties back to the DIC post- I made extensive notes + concept).
Still confused?? Mug up this info:
BT = platelets problem
PT/INR = extrinsic (warfarin) problem
aPTT = intrinsic (heparin) pathway problem
TT = fibrinogen problem
Direct vs indirect Coombs
Same reagent in both: anti-human globulin.
The only difference is where the antibody is sitting.
Direct Coombs: antibody is already stuck on the patient's RBCs. Wash the cells, add AHG, watch them clump. AIHA, HDFN, haemolytic transfusion reaction
Indirect Coombs: antibody is still floating in the patient's serum. Incubate serum with reagent RBCs first, then add AHG. Antenatal antibody screen, crossmatch.
Direct asks: is the cell already coated?
Indirect asks: is the plasma still loaded?
Low C3 + low C4 vs low C3 + normal C4
Low C3 with low C4 vs low C3 with normal C4 confuses a lot of PG aspirants.
Stop memorising disease lists. Ask one question: was C4 used?
Classical pathway: C1 cleaves C4 and C2 before it ever reaches C3. Immune complexes burn both. SLE, cryoglobulinemia.
Alternative pathway: skips C4 completely and chews C3 directly. PSGN, C3 glomerulopathy.
C4 is the fingerprint. It tells you which door the fire came through.