**Question:** Inward flow of Na+ in heart leads to:

A. Hyperpolarization
B. Depolarization
C. Action potential prolongation
D. Increased contractility

**Core Concept:** The inward flow of sodium ions (Na+) plays a crucial role in the generation and maintenance of the cardiac action potential. In the heart, the inward flow of Na+ is primarily due to the opening of voltage-gated Na+ channels, which occurs during the plateau phase of the action potential.

**Why the Correct Answer is Right:** Inward flow of Na+ during the plateau phase leads to hyperpolarization of the cardiac cell membrane, which is necessary for the repolarization process to commence. This hyperpolarization results in the inactivation of voltage-gated Na+, K+, and Ca2+ channels, allowing the cell membrane to regain its resting membrane potential.

**Why Each Wrong Option is Incorrect:**

A. Depolarization: The correct answer states that inward flow of Na+ leads to hyperpolarization, not depolarization. Depolarization is caused by the outward flow of K+ ions during the plateau phase.

B. Action potential prolongation: Inward flow of Na+ actually contributes to repolarization, not prolongation of the action potential. Prolongation occurs due to the delayed rectifier K+ channels, which are activated during the plateau phase.

C. Increased contractility: While Na+ ions are involved in the action potential generation, they do not directly influence cardiac contractility. Contractility is primarily regulated by calcium ions (Ca2+) and the sarcoplasmic reticulum.

D. Sodium ion concentration gradient: The question specifically asks about the effect of Na+ flow on the cardiac action potential, not the concentration gradient. The concentration gradient indeed exists, but the correct answer focuses on the functional consequences of Na+ flow on membrane potential.

**Clinical Pearl:** Understanding the role of Na+ ions in the cardiac action potential is crucial for understanding cardiac physiology and pathology, particularly in relation to arrhythmias and conduction disorders. A thorough understanding of ionic currents and their regulation is essential for clinical practice in cardiology.