Dr. Kyriacou commends you on your excellent management so far. As a dihydropyridine calcium channel blocker, amlodipine reduces afterload through arterial vasodilation and improves coronary vasomotion, which lowers myocardial oxygen demand without further slowing sinus rate or atrioventricular conduction. This makes it a safe and effective partner with bisoprolol for symptom control.  

Further developments 

Mr. Garcia is celebrating his 30th wedding anniversary with his wife by going away to a resort for the weekend. Two days ago, Mr. Garcia confided in one of his friends that he was concerned about erectile dysfunction. His friend told him that he could give him some of his prescription sildenafil, which Mr. Garcia took that evening. Mr. Garcia also took all his regular medication that day as well (aspirin, bisoprolol, amlodipine, simvastatin). He also took glyceryl trinitrate as he was concerned about his angina exertion. 

Later that evening, Mr. Garcia starts feeling extremely lightheaded and collapses. At the Emergency Department, his blood pressure is found to be very low. 

Which drug, taken by Mr. Garcia, is most likely to have synergistic effects with sildenafil and may account for Mr. Garcia’s hypotension?