Acidic urine favors which types of stones?

• A. Uric acid stones
• B. Calcium phosphate and struvite stones
• C. Uric acid and cystine stones
• D. Calcium oxalate stones

Answer: (C)

When urine is acidic, the solubility of certain stone-forming substances decreases, making crystal formation more likely. Uric acid is less soluble in low pH urine, so uric acid stones form more readily in acidic conditions. Cystine also has very low solubility in acidic urine, and stones from this amino acid tend to appear when the urine is acidic. In contrast, calcium phosphate and struvite stones prefer alkaline urine, and calcium oxalate stones can form over a wide pH range and are not specifically tied to acidity. Therefore, acidic urine favors uric acid stones and cystine stones.