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
A male presented to the emergency depament with renal calculi. Extracorporeal shock wave lithotripsy was suggested after a detailed examination. But the procedure failed. Which of the following type of stone is MOST resistant to lithotripsy? 
Select and explain. 
A. Ca oxalate, B. Triple phosphate, C. Uric acid, D. Cystine
D: Cystine
Answer: D: Cystine 
Explanation: Cystine lithiasis: Cystine lithiasis is secondary to an inborn error of metabolism resulting in abnormal intestinal (small bowel) mucosal absorption and renal tubular absorption of dibasic amino acids, including cystine, ornithine, lysine, and arginine. The solubility of cystine is pH-dependent, with a pK of approximately 8.1. There is no known inhibitor for cystine calculi, and cystine stone formation is completely dependent on excessive cystine excretion. Radiographic appearance of a faintly opaque, ground-glass, smooth-edged stone. Surgical treatment is similar to that for other stones except that most stones are recalcitrant to extracorporeal shock wave lithotripsy (ESWL). Ref: Stoller M.L. (2008). Chapter 16. Urinary Stone Disease. In E.A. Tanagho, J.W. McAninch (Eds), Smith's General Urology, 17e.