This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
Absolute indication for surgery in hyperparathyroidism is 
Select and explain. 
A. Serum calcium < 2.85mmol/lit, B. Increased bone density, C. Urinary tract calculi, D. Patient more than 50 years of age
C: Urinary tract calculi
Answer: C: Urinary tract calculi 
Explanation: None