Which urine pH pairing is correct for stone formation?

• A. Uric acid stones form in acidic urine; calcium phosphate stones form in alkaline urine
• B. Uric acid stones form in alkaline urine; calcium phosphate stones form in acidic urine
• C. Both stones form best in neutral urine
• D. Uric acid stones form regardless of pH

Answer: (A)

Urine pH influences the solubility of different stone-forming minerals, guiding which crystals tend to form. Uric acid stones are favored by acidic urine because uric acid becomes poorly soluble at low pH and tends to precipitate; when the urine is more alkaline, uric acid dissolves more readily, reducing stone risk. Calcium phosphate stones, on the other hand, precipitate more readily in alkaline urine because phosphate salts combine with calcium at higher pH to form calcium phosphate compounds. So pairing uric acid stones with acidic urine and calcium phosphate stones with alkaline urine reflects how pH drives their formation. Other stones have different pH associations: calcium oxalate can form across a range of pH, while struvite stones form in alkaline urine typically due to infection with urease-producing organisms, and cystine stones tend to form in acidic urine.