Which stone type occurs in acidic urine and is often associated with high purine intake?

• A. Urinary uric acid stones.
• B. Calcium oxalate stones.
• C. Struvite stones.
• D. Cystine stones.

Answer: (A)

This question tests how urine acidity and dietary purines influence stone formation. Uric acid stones form most readily in acidic urine because uric acid is poorly soluble when the pH is low, so it tends to precipitate. When purines in the diet are high, more uric acid is produced from purine metabolism, increasing the amount excreted in urine and the likelihood of crystallization as stones. So a stone type that appears in acidic urine and is linked to high purine intake is a urinary uric acid stone.

Calcium oxalate stones are the most common overall and can form across a range of urine pH, so they aren’t specifically tied to acidic urine or high purine intake. Struvite stones arise in alkaline urine due to infection with urease-producing bacteria, not diet. Cystine stones come from a genetic disorder (cystinuria) and aren’t specifically related to purine intake either.