<prompt> You decide not to prescribe sildenafil in a patient because the patient told you that he is taking an antianginal drug. Which of the following can it be?Calcium channel blockersb adrenergic blockersOrganic nitratesAngiotensin conveing enzyme inhibitors  <response>: Nitrates act by increasing cGMP and sildenafil inhibits the breakdown of this compound by inhibiting phosphodiesterase. Marked increase in cGMP levels may result in the profound hypotension and reflex tachycardia. 