**Question:** P wave in ECG is due to:

A. Atrial depolarization
B. Ventricular depolarization
C. Ventricular repolarization
D. Sinus node activity

**Core Concept:**
The P wave in an electrocardiogram (ECG) represents atrial depolarization and is a manifestation of atrial electrical activity. The atria are responsible for pumping blood into the ventricles, and the P wave occurs before the QRS complex, which represents ventricular depolarization during each cardiac cycle.

**Why the Correct Answer is Right:**
The correct answer, A, "Atrial depolarization," explains the origin and cause of the P wave. During the cardiac cycle, depolarization occurs in the atria first, followed by depolarization in the ventricles (option B). This sequence results in the P wave preceding the QRS complex.

**Why Each Wrong Option is Incorrect:**
B. Ventricular depolarization is incorrect because it occurs after the P wave, following atrial depolarization. Ventricular depolarization is responsible for the QRS complex, not the P wave.
C. Ventricular repolarization is incorrect because it occurs after the QRS complex, following ventricular depolarization. It does not result in the P wave.
D. Sinus node activity is incorrect because it is related to the sinoatrial (SA) node, responsible for regulating heart rate and initiating the cardiac cycle. The P wave is due to atrial depolarization, not SA node activity.

**Clinical Pearl:**
Understanding the cardiac cycle and the sequence of events in the ECG helps medical students and practitioners interpret ECG findings accurately. The P wave, QRS complex, and T wave provide essential information about the heart's electrical activity and can assist in diagnosing various cardiac disorders.