Hall Anesthesia Practice Test

The key idea is phosphodiesterase inhibitors. Sildenafil blocks the PDE5 enzyme, which prevents the breakdown of cGMP in smooth muscle, leading to increased cGMP and vasodilation (notably in the penis and pulmonary vasculature). Milrinone blocks PDE3, raising cAMP in cardiac and vascular smooth muscle, which produces positive inotropic effects and vasodilation. Although they inhibit different PDE isoforms, they belong to the same broad drug family because both impede phosphodiesterase enzymes to boost cyclic nucleotide signaling. The other drugs act via entirely different mechanisms—yohimbine on adrenergic receptors, nitroglycerin by NO donation to increase cGMP (not by PDE inhibition), and enalapril by blocking the renin-angiotensin system.