This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Pharmacology
The biochemical mechanism of action of digitalis is associated with: 
Select and explain. 
A. An increase in conduction from atrium to ventricle, B. An increase in ATP synthesis, C. An increase in systolic intracellular calcium levels, D. A block of calcium channels
C: An increase in systolic intracellular calcium levels
Answer: C: An increase in systolic intracellular calcium levels 
Explanation: Digoxin is a cardiac glycoside. It directly inhibits Na-K ATPase and increases intracellular Na and decrease the extrusion of calcium. Thus increases intracellular Calcium. By increasing the calcium content and sodium content, force of contraction increases. (Ref-Essential of medical pharmacology K D TRIPATHI 6 Edition)