This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Pharmacology
[{'content': 'Methanamine salts are used as urinbary antiseptics. The reason theylack systemic antibacterial action is that they are: \nSelect and explain. \nA. Not absorbed into systemic circulation after oral use, B. Rapidly metabolized by liver drug metabolizing enzymes, C. Conveed to formaldehyde at low urinary pH, D. Substrates for active tubular secretion', 'role': 'user'}, {'content': 'Answer: C \nExplanation: (Ref: KDT 6/e p735) Methanamine mandelate is used as an urinary antiseptic. It acts by conversion to formaldehyde at acidic urinary pH. Mandelic acid formed from this salt also serves the function of urinary acidifying agent. These are not effective against proteus because proteus results in the formation of ammonia which alkalinizes the urine.', 'role': 'assistant'}]