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
Which of the following are hardest renal stones 
Select and explain. 
A. Calcium Oxalate, B. Struvite, C. Xanthine, D. Cysteine
A: Calcium Oxalate
Answer: A: Calcium Oxalate 
Explanation: Answer- A. Calcium OxalatePure calcium phosphate stones and calcium monohydrate stones are the densest per volume ofstone.Among the given options calcium oxalate stones are densest (hardest).