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: Pharmacology
Methenamine salts are used as urinary antiseptics. The reason they lack systemic antibacterial action is that they are : 
Select and explain. 
A. Not absorbed into systemic circulation after oral use, B. Rapidly metabolized by liver drug metabolizing enzymes, C. Converted to formaldehyde at low urinary pH, D. Substrates for active tubular secretion
C: Converted to formaldehyde at low urinary pH
Answer: C: Converted to formaldehyde at low urinary pH 
Explanation: None