This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary bladder, Subject name: Surgery
The commonest bladder stone is: 
Select and explain. 
A. Triple phosphate, B. Xanthine, C. Uric acid, D. Cysteine
C: Uric acid
Answer: C: Uric acid 
Explanation: Bladder stone Primary bladder stone- Aka Endemic bladder calculi- Most common type of primary bladder stone - Ammonium urate - other stones- Calcium oxalate. Secondary bladder stone - More common in elderly males due to BPH /Bladder outlet obstruction- Most common type of secondary bladder stone - Uric acid> Struvite- Overall secondary bladder stone - most common Investigations- Investigation of choice- Ultrasound Treatment- For small stones - cystolitholapexy- For Large stones* Electrohydraulic lithotripsy* Suprapubic cystolithotomy