A 73-year-old woman with systolic heart failure and chronic atrial fibrillation presents with dyspnea. Vitals: BP 98/62, HR 102, RR 20, SpO2 92%. She is prescribed digoxin for symptom improvement. Recent echocardiography shows ejection fraction of 28%. Serum potassium is 4.2 mEq/L. She denies chest pain. By inhibiting the sodium-potassium ATPase in cardiac myocytes, which intracellular change most directly increases contractility?

1. A)Decreased intracellular cyclic AMP in myocytes
2. B)Increased potassium efflux during phase 3 repolarization
3. C)Increased intracellular calcium due to reduced sodium calcium exchangeGABARITO
4. D)Blockade of funny current in the sinoatrial node
5. E)Activation of guanylyl cyclase in vascular smooth muscle