Digoxin exerts its therapeutic effect by inhibiting which cardiac enzyme?

• A. Acetylcholinesterase inhibition
• B. Phosphodiesterase inhibition
• C. Monoamine oxidase inhibition
• D. Inhibiting Na+/K+-ATPase

Answer: (D)

Digoxin works by inhibiting the Na+/K+-ATPase pump on cardiac muscle cells. This pump keeps intracellular sodium low; when it’s blocked, intracellular Na+ rises, which reduces the activity of the Na+/Ca2+ exchanger that normally removes Ca2+ in exchange for Na+. With less Ca2+ being expelled, intracellular calcium increases, enhancing the strength of each heartbeat (positive inotropy). At the same time, digoxin boosts vagal tone, slowing conduction through the AV node and helping control certain arrhythmias. The other enzymes listed—acetylcholinesterase, phosphodiesterase, and monoamine oxidase—are not the targets of digoxin and would produce different effects if inhibited. So the therapeutic effect is best explained by inhibition of Na+/K+-ATPase.