StepWise USMLE
renal & urinary pathology · systems

Kidney & Urinary Tract
Pathology

Congenital anomalies · cystic diseases · glomerular & tubulointerstitial disorders · tumors · obstructive uropathy

🧬 Congenital anomalies

  • Renal agenesis
    • Bilateral: incompatible with life; oligohydramnios → Potter facies, clubfoot, pulmonary hypoplasia
    • Unilateral: compensatory hypertrophy; usually asymptomatic
  • Hypoplasia – decreased calyces/lobes; unilateral, normal weight not achieved
  • Horseshoe kidney – fusion at lower pole; ↑ risk calculi; normal function
  • Ectopic kidney – most common pelvic; ureter tortuosity → pyelonephritis risk
  • Oligohydramnios sequence: renal agenesis → oligohydramnios → compression → facial/limb anomalies

🫘 Cystic diseases

  • ARPKD (infantile): PKHD1; bilateral enlarged spongy kidneys; hepatic cysts; pulmonary hypoplasia
  • ADPKD (adult): PKD1 (polycystin 1) or PKD2; normal function until middle age; HTN, stones, hematuria; extrarenal: liver cysts, berry aneurysms, MVP, diverticula
  • Renal dysplasia – most common cystic in children; cartilage & immature ducts
  • Acquired – dialysis patients; risk adenoma/RCC; simple retention cysts common
  • Medullary sponge – nephrolithiasis; mostly benign
  • Nephronophthisis – AR; polyuria/polydipsia → CKD in young adults
  • ADPKD: <10% nephrons involved initially; cysts expand and compress parenchyma → delayed dysfunction

🩸 Glomerular diseases

NephriticNephrotic
hematuria (RBC casts)proteinuria >3.5g/d
hypertension, azotemiahypoalbuminemia, edema
oliguria, proteinuria <3.5ghyperlipidemia, lipiduria
  • APSGN – post-streptococcal; immune complex; low C3; granular IgG/C3; subepithelial "humps"; children >95% recover
  • RPGN – crescents; anti-GBM (Goodpasture: pulmonary hemorrhage); linear IgG; pauci-immune ANCA+
  • IgA nephropathy – most common; recurrent hematuria post-URI; mesangial IgA; slow progression
  • MPGN – type I subendothelial; type II dense deposit (C3 nephritic factor); tram-tracking; ↓C3
  • Alport – X-linked COL4A5; hematuria, sensorineural deafness, ocular; basketweave EM
  • Membranous – adults; autoantibodies; spikes (silver); subepithelial deposits; 85% idiopathic
  • Minimal change – children; foot process effacement; steroid-responsive; no immune deposits
  • FSGS – focal/segmental; sclerosis; IgM/C3; poor steroid response; collapsing variant worse

🧪 Tubulointerstitial

  • Acute pyelonephritis – ascending infection (E. coli, Proteus); WBC casts, CVA tenderness; abscesses
  • Chronic pyelonephritis – scarring, calyceal blunting; thyroidization; progressive renal insufficiency
  • Drug-induced – penicillins, NSAIDs, diuretics; fever, eosinophilia, rash; recovery after withdrawal
  • Urate nephropathy – urate crystals (leukemia, gout, lead); acute renal failure
  • Fever + CVA tenderness + WBC casts = pyelonephritis (vs cystitis)

⚡ Acute tubular injury

  • Most common cause of acute renal failure in the US
  • Oliguria, ↑BUN/Cr, metabolic acidosis, hyperkalemia; muddy brown casts
  • Ischemic – hemorrhage, hypotension, shock, vasoconstriction
  • Toxic – aminoglycosides, contrast, heavy metals, ethylene glycol, myoglobin
  • Prognosis excellent if patient survives underlying cause and no preexisting CKD

🪨 Urolithiasis

  • Affects ~6%, male > female
  • Calcium oxalate (75%) – radiopaque; alkaline urine
  • Struvite (MgNH4PO4) – urease-splitting bacteria (Proteus); staghorn calculi
  • Uric acid – gout, leukemia; acidic urine
  • Cystine – uncommon; acidic urine
  • Renal colic, hematuria, obstruction; lithotripsy/endoscopic removal

🔄 Chronic kidney disease

  • End stage of many renal diseases; bilaterally shrunken
  • Irreversible azotemia, normocytic anemia, platelet dysfunction, renal osteodystrophy, HTN

🫀 Vascular disorders

  • Renal artery stenosis – atheroma (most common); fibromuscular dysplasia (string of beads); secondary HTN
  • Benign nephrosclerosis – hypertensive; fine granular surface; hyaline arteriolosclerosis
  • Malignant HTN – fibrinoid necrosis, hyperplastic arteriolosclerosis; flea-bitten appearance; acute renal failure
  • Renal infarction – thromboemboli; flank pain, hematuria
  • Sickle cell – medullary infarcts, papillary necrosis, pyelonephritis
  • Diffuse cortical necrosis – obstetric emergencies, DIC; anuria

🧫 Tumors

  • Renal cell carcinoma – 50-70y, male>female; smoking, VHL; classic triad (hematuria, mass, flank pain); paraneoplastic (erythropoietin, renin, PTH, ACTH, gonadotropins)
  • Clear cell (most common; 3p loss; VHL); papillary (multifocal, chromosome 7/17 gains); chromophobe (least aggressive)
  • Wilms tumor – 2-5y; WAGR, DDS, BWS; WT1/WT2; blastema, epithelium, stroma; surgery+chemo+RT → 90% survival
  • Benign – cortical adenoma, angiomyolipoma (tuberous sclerosis), oncocytoma

🚧 Obstructive uropathy

  • Hydronephrosis – dilation of ureter & pelvis; stones, BPH, retroperitoneal fibrosis, cervical cancer
  • Unilateral → kidney may enlarge to 20 cm; sudden complete → papillary necrosis
  • Bilateral complete → anuria; incomplete → polyuria

🔗 Ureteral disorders

  • Congenital: double ureters, megaureter
  • UPJ obstruction – most common cause of hydronephrosis in children
  • Ureteritis cystica – chronic inflammation, mucosal cysts; adenocarcinoma risk
  • Retroperitoneal fibrosis – idiopathic; entraps ureters; IgG4-associated
  • Transitional cell carcinoma – most common ureteral malignancy

🧴 Bladder pathology

  • Exstrophy – open bladder; urachal remnants → adenocarcinoma
  • Cystitis – E. coli, Proteus; radiation, cyclophosphamide; frequency, dysuria, suprapubic pain
  • Cystocele – bladder prolapse; acquired diverticula from obstruction
  • Transitional cell carcinoma – most common; smoking; CIS → invasive; papillary precursors
  • Squamous cell – Schistosoma haematobium (Middle East)
  • Adenocarcinoma – glandular metaplasia, cystitis cystica
  • Malakoplakia – macrophage defect; Michaelis-Gutmann bodies