This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Pharmacology
In terms of the ability of drugs like digoxin to increase cardiac contractility, their primary action on cardiac cells is 
Select and explain. 
A. activation of adenylyl cyclase, B. inactivation of Na channels, C. activation of the slow Ca channel, D. inhibition of Na/K-ATPase
D: inhibition of Na/K-ATPase
Answer: D: inhibition of Na/K-ATPase 
Explanation: Cardiac glycosides increase contractility by inhibiting the Na/K-ATPase pump, causing an increase in intracellular Na. This, in turn, increases intracellular Ca by slowing down Na/Ca exchange. The increase in intracellular Ca leads to its binding to the troponin-tropomyosin complex, causing an allosteric change and facilitating the interaction between actin and myosin.