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
Long term use of nitrates lead to decreased effect because of - 
Select and explain. 
A. Development of resistance, B. SH group in the enzyme, C. Decreased oral absorption, D. Increased resistance
B: SH group in the enzyme
Answer: B: SH group in the enzyme 
Explanation: Ans. is 'b' i.e., SH groups in the enzyme o Tolerance develops to hemodynamic and antiischaemic effects of nitrates if they are continuously present in the body. No significant tolerance develops when nitrates are used intermittently. However; significant tolerance develops when nitrates are used continuously. This mechanism of nitrate tolerance is not well understood. The mechanism proposed is --> "Reduced ability to generate NO due to depletion of cellular SH radicals".