This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Antimetabolites and Quinolones, Subject name: Pharmacology
Ratio of sulfamethoxazole and trimethoprim in cotrimoxazole tablet is:- 
Select and explain. 
A. 5:01, B. 1:05, C. 1:01, D. 5:05
A: 5:01
Answer: A: 5:01 
Explanation: COTRIMOXAZOLE This is a fixed dose combination of sulfamethoxazole and trimethoprim in a ratio of 5:1. Due to different bioavailability (more for sulfamethoxazole), plasma concentration of the two drugs attained is 20:1 (in vivo), which is the best bactericidal concentration required. The bactericidal activity is due to sequential blockade at two steps in the DNA synthesis (sulfamethoxazole inhibits folate synthase and trimethoprim inhibits DHFRase). It is the drug of choice for pneumocystosis, nocardiosis and infections caused by Burkholderia cepacia.