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: Medicine
Magnesium-ammonium phosphate stones are usually secondary to urinary tract infection with which of the following: 
Select and explain. 
A. Escherichia coli, B. Proteus species, C. Klebsiella, D. Enterococcus
B: Proteus species
Answer: B: Proteus species 
Explanation: Magnesium-ammonium-phosphate stones are usually secondary to urinary tract infection with bacteria that produce urease (primarily Proteus species). The urease hydrolyzes the urea producing ammonia and thereby raising the pH as well as providing the source of ammonia. Eradication of the infection prevents fuher stone formation. After calculi removal, prevention of stone growth is best accomplished by urinary acidification, long-term use of antibiotics, and the use of acetohydroxamic acid (a urease inhibitor that maintains an acid urinary pH). Ref: Obrador G.T. (2009). Chapter 17. Chronic Renal Failure & the Uremic Syndrome. In E.V. Lerma, J.S. Berns, A.R. Nissenson (Eds), CURRENT Diagnosis & Treatment: Nephrology & Hypeension.