This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urology, Subject name: Surgery
Prostatic calculi is made up of ? 
Select and explain. 
A. Calcium carbonate, B. Calcium oxalate, C. Calcium phosphate, D. Triple phosphate
C: Calcium phosphate
Answer: C: Calcium phosphate 
Explanation: Prostatic calculi are of two varieties: endogenous, which are common, and exogenous, which are comparatively rare. An exogenous prostatic calculus is a urinary (commonly ureteric) calculus that becomes arrested in the prostatic urethra. Endogenous prostatic calculi are usually composed of calcium phosphate combined with about 20% organic material.Reference : page 1353 Bailey and Love's sho practice of surgery 25th edition