A 16-year-old boy presents with persistent microscopic hematuria detected on routine urinalysis. Audiometry shows bilateral symmetric sensorineural hearing loss beginning at 2 kHz and sloping downward, as marked **A** in the diagram. Renal biopsy electron microscopy reveals alternating thinning and thickening of the glomerular basement membrane with a characteristic basket-weave appearance. Which of the following best explains the pathophysiology of this patient's hearing loss?

Explanation

## Why option 1 is right Alport syndrome is a hereditary glomerular basement membrane disorder caused by mutations in genes encoding type IV collagen (COL4A3, COL4A4, or COL4A5). Type IV collagen forms the structural backbone of basement membranes throughout the body, including the cochlear basilar membrane. Defective collagen in the cochlea leads to the characteristic bilateral, symmetric, high-frequency sensorineural hearing loss (beginning at 2 kHz and sloping downward) that appears in late childhood/adolescence and progresses with age. The same collagen defect affects the glomerular basement membrane (producing the basket-weave appearance on electron microscopy) and the ocular lens capsule, forming the classic triad of Alport syndrome: progressive hematuric nephritis, bilateral high-frequency SNHL, and ocular abnormalities. (Harrison 21e Ch 311; Dhingra ENT 7e Ch 18) ## Why each distractor is wrong - **Option 2 (Autoimmune destruction)**: Anti-GBM antibodies cause post-transplant Goodpasture-like nephritis in Alport patients, not the primary hearing loss. The hearing loss in Alport is structural (collagen defect), not immune-mediated. - **Option 3 (Hypertension-induced ischemia)**: While hypertension accompanies CKD progression in Alport, it is not the primary cause of the high-frequency SNHL. The hearing loss is present even in early stages before significant hypertension develops, and it reflects the underlying collagen defect in the cochlea. - **Option 4 (Aminoglycoside ototoxicity)**: Aminoglycosides are contraindicated in Alport syndrome and should be avoided as nephrotoxins. They are not the cause of the hearing loss; the hearing loss precedes and is independent of antibiotic use. **High-Yield:** Alport syndrome = hematuria + high-frequency SNHL + anterior lenticonus; all three features result from defective type IV collagen in basement membranes (GBM, cochlea, eye). Start ACEi/ARB at first sign of proteinuria to delay ESRD. [cite: Harrison 21e Ch 311; Dhingra ENT 7e Ch 18]