**Question:** A 4-year-old girl is brought to the pediatrician's office. Her father reports that she suddenly became pale and stopped running while he had been playfully chasing her and her pet Chihuahua. After 30 minutes, she was no longer pale and wanted to resume the game. She has never had a previous episode and has never been cyanotic. Her physical examination was normal, as were her chest x-ray and echocardiogram. An ECG showed the pattern seen on the next page, which indicates which of the following?

A. Premature ventricular contractions (PVCs)
B. Supraventricular tachycardia (SVT)
C. Ventricular tachycardia (VT)
D. Atrial tachycardia (AT)

**Correct Answer:**

**Core Concept:** The ECG pattern described in the question is a common feature of **supraventricular tachycardia (SVT)**, a type of rapid heart rate originating from the upper chambers of the heart (atria or AV node).

**Why the Correct Answer is Right:** The ECG pattern seen in SVT typically demonstrates a wide QRS complex and a rapid heart rate, which is evident in this case. Additionally, no specific rhythm disturbances like premature ventricular contractions (PVCs), ventricular tachycardia (VT), or atrial tachycardia (AT) are mentioned in the question. Therefore, the correct answer is B.

**Why Each Wrong Option is Incorrect:**

A. Premature ventricular contractions (PVCs): PVCS are single premature ventricular beats that occur outside the normal heart rhythm. The pattern described in the question is not consistent with a single premature ventricular contraction due to the wide QRS complex and rapid heart rate.

B. Supraventricular tachycardia (SVT): SVT is the correct answer, as it is a type of rapid heart rate originating from the upper chambers of the heart (atria or AV node). The ECG pattern described matches SVT.

C. Ventricular tachycardia (VT): VT refers to arrhythmias that originate from the ventricles, which are the lower chambers of the heart. The described ECG pattern does not match VT.

D. Atrial tachycardia (AT): AT refers to arrhythmias originating from the atria, which are the upper chambers of the heart. The ECG pattern does not match AT.

**Clinical Pearls:**

Supraventricular tachycardia (SVT) is a common pediatric arrhythmia and is usually benign. It occurs due to re-entrant pathways within the atrioventricular node or atria. Understanding the specific type of SVT (e.g., atrioventricular nodal reentrant tachycardia or atrioventricular reciprocating tachycardia) is essential for accurate diagnosis and appropriate management.