Depolarisation of atria on ECG is seen as
**Core Concept**
The question is testing the understanding of the relationship between atrial depolarization and its representation on an electrocardiogram (ECG). Atrial depolarization is the electrical impulse that spreads through the atria, leading to muscle contraction. This is a critical concept in understanding cardiac electrophysiology.
**Why the Correct Answer is Right**
Atrial depolarization is represented on an ECG as a small, upward deflection known as the P wave. This occurs due to the depolarization of the atrial myocardium, which is triggered by the sinoatrial (SA) node. The P wave is a result of the electrical impulse spreading through the atria, causing them to contract. The P wave is typically seen before the QRS complex, which represents ventricular depolarization.
**Why Each Wrong Option is Incorrect**
* **Option A:** Atrial depolarization is not represented by the T wave, which is associated with ventricular repolarization. The T wave is a downward deflection that occurs after the QRS complex, indicating the recovery of the ventricles.
* **Option B:** The QRS complex is actually associated with ventricular depolarization, not atrial depolarization. The QRS complex is a large, downward deflection that represents the electrical impulse spreading through the ventricles.
* **Option C:** The U wave is a small, positive deflection that is sometimes seen after the T wave, but it is not directly related to atrial depolarization. The U wave is thought to represent the late repolarization of the ventricles.
**Clinical Pearl / High-Yield Fact**
The P wave is a critical component of the ECG, as it can provide valuable information about atrial function and the presence of conditions such as atrial fibrillation.
**Correct Answer: B. QRS complex. Ventricular depolarization.**