All subjects
Renal & Urinary
Clearance, acid-base, glomerular disease and diuretics — the highest-yield calculation-heavy system.
Exam weight 5–7% · 2 QBank items · 4 flashcards
Renal physiology
- GFR is estimated with inulin or creatinine clearance; effective renal plasma flow with PAH; filtration fraction = GFR/RPF.
- Efferent arteriolar constriction by angiotensin II raises GFR and filtration fraction; NSAIDs constrict the afferent arteriole and drop GFR.
- Proximal tubule reabsorbs 65–80% of sodium plus all glucose and amino acids; Fanconi syndrome loses all of them.
- Free water clearance and urine osmolality distinguish SIADH (concentrated urine) from diabetes insipidus (dilute urine).
Acid-base
- Anion gap = Na − (Cl + HCO₃); elevated in MUDPILES (methanol, uremia, DKA, propylene glycol, iron/INH, lactate, ethylene glycol, salicylates).
- Winter formula: expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2 — deviation means a second disorder.
- Renal tubular acidosis type 1 (distal) has urine pH above 5.5 with stones; type 2 (proximal) has bicarbonate wasting and rickets; type 4 has hyperkalemia from hypoaldosteronism.
- Vomiting causes hypochloremic, hypokalemic metabolic alkalosis with paradoxical aciduria.
Glomerular disease
- Nephrotic: proteinuria above 3.5 g/day, hypoalbuminemia, edema, hyperlipidemia, loss of antithrombin III → thrombosis.
- Minimal change disease in children responds to steroids; effacement of foot processes on electron microscopy only.
- Membranous nephropathy: anti-PLA2R, subepithelial spikes, associated with hepatitis B, solid tumours and NSAIDs.
- Nephritic: IgA nephropathy follows mucosal infection within days; post-streptococcal follows 2–4 weeks later with low C3.
AKI, CKD & stones
- Prerenal AKI: BUN/Cr above 20, FENa below 1%; intrinsic (ATN): FENa above 2% with muddy brown casts.
- CKD causes anemia (low erythropoietin), hyperphosphatemia, hypocalcemia and secondary hyperparathyroidism with renal osteodystrophy.
- Calcium oxalate stones are the most common and are radiopaque; struvite stones form staghorn calculi with urease-positive organisms.
- Uric acid stones are radiolucent and form in acidic urine — treat with urinary alkalinisation.
Diuretics
- Loop diuretics inhibit NKCC2: ototoxicity, hypokalemia, hypocalcemia, gout.
- Thiazides inhibit the NCC: hypercalcemia, hyperglycemia, hyperlipidemia, hyperuricemia, hyponatremia.
- Spironolactone blocks the mineralocorticoid receptor: hyperkalemia and gynecomastia; eplerenone avoids the endocrine effects.
- Acetazolamide causes a proximal, hyperchloremic metabolic acidosis — used for glaucoma and altitude sickness.