A 20-year-old man passes a spiky, radio-opaque stone. Stone analysis reveals hexagonal crystals. Urinalysis shows increased cystine, lysine, arginine, and ornithine. The primary defect lies in:
- A Cystathionine beta-synthase
- B Hepatic cystine reductase
- C Renal tubular reabsorption of dibasic amino acids ✓
- D Renal tubular secretion of uric acid
Correct answer: C. Renal tubular reabsorption of dibasic amino acids
Explanation
Cystinuria is an autosomal recessive defect of the epithelial dibasic amino acid transporter (subunit rBAT, SLC3A1, or b0,+AT, SLC7A9) in the proximal tubule and intestine. Cystine, lysine, arginine and ornithine are poorly reabsorbed; only cystine is insoluble, precipitating as hexagonal crystals and staghorn or spiky stones. Cystathionine beta-synthase deficiency is homocystinuria, and no clinically relevant hepatic cystine reductase exists, making option B a distractor.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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