**Question:** Plateau of cardiac action potential is due to

A. Depolarizing current from Na+ channels
B. Hyperpolarizing current from K+ channels
C. Depolarizing current from Ca2+ channels
D. Depolarizing current from L-type Ca2+ channels

**Core Concept:**
The cardiac action potential is a series of electrical signals that allow the heart to contract rhythmically and efficiently. The cardiac action potential is divided into four phases: resting potential, upstroke (phase 0), plateau (phase 2), and repolarization (phase 3). The plateau phase is crucial for maintaining the contraction of the heart muscle.

**Why the Correct Answer is Right:**
The plateau phase of the cardiac action potential is primarily due to the depolarizing current from L-type calcium channels (D). During this phase, the influx of calcium ions (Ca2+) through L-type calcium channels leads to an increase in the membrane potential, maintaining the heart muscle contraction. This is in contrast to the other options, which describe processes that do not contribute significantly to the plateau phase.

**Why Each Wrong Option is Incorrect:**
A. Depolarizing current from Na+ channels (option A) is responsible for the initial rapid phase of the action potential (phase 0) but not the plateau phase.
B. Hyperpolarizing current from K+ channels (option B) counteracts the depolarization during the plateau phase but does not cause it.
C. Depolarizing current from Ca2+ channels (option C) only contributes to the early phase of the action potential, not the plateau phase.
D. Depolarizing current from L-type calcium channels (option D) is the correct answer, as it plays a crucial role in maintaining the plateau phase and heart contraction.

**Clinical Pearl:**
Understanding the plateau phase of the cardiac action potential is essential for understanding the pathophysiology of cardiac arrhythmias, such as long QT syndrome caused by defects in L-type calcium channels. This knowledge helps in diagnosing and treating cardiac disorders effectively.