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
[{'content': 'Which of the following stones is hard to break by ESWL? \nSelect and explain. \nA. Calcium Oxalate Monohydrate, B. Calcium Oxalate Dihydrate, C. Uric acid, D. Struvite', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Calcium oxalate monohydrate and Cystine stones are difficult to fragment by ESWL. Other factors which contribute to poor outcome in ESWL are, obesity, stones larger than 2cm, stone within the calyces. Ref: Hand Book of Surgical Consent By Rajesh Nair, David J. Holroyd, 1st Edition, Page 302', 'role': 'assistant'}]