This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney & Ureturs, Subject name: Surgery
[{'content': 'Radiolucent stone is \nSelect and explain. \nA. Oxalate calculus, B. Phosphate calculus, C. Uric acid calculus, D. Cysteine calculi', 'role': 'user'}, {'content': 'Answer: C \nExplanation: (Uric acid calculus) (1317-LB) (1296-B &L 25th)* About 90% of calculi are radiopaque (calcium, phosphate oxalates, cystine)* Uric acid stones are radiolucent and appear on an excretion urogram as a filling defect* Spiral CT has become the study of choice in emergent situations, as the entire urinary tract can be scanned rapidly and without contrast injection. (1042-CSDT 11th)', 'role': 'assistant'}]