Okay, let's tackle this question. The patient is a 3-4 month old baby with a heart rate of 250 beats per minute. The ECG shows a QRS complex less than 0.07 seconds and no P waves. The question is asking for the diagnosis.

First, I need to recall the different types of supraventricular tachycardias (SVTs) in infants. The high heart rate suggests a tachycardia. The QRS duration being less than 0.07 seconds means it's narrow complex, so it's not ventricular tachycardia. The absence of P waves is key here. In sinus tachycardia, P waves would be present but maybe buried in the QRS or T waves. Atrial flutter or fibrillation might show abnormal P waves, but in an infant, those are less common.

The classic scenario for an infant with SVT is atrioventricular nodal reentrant tachycardia (AVNRT) or atrioventricular reentrant tachycardia (AVRT). However, in AVNRT, there are usually retrograde P waves that might not be visible if they're buried in the QRS or T wave. But the question states there are no P waves. Another possibility is junctional tachycardia, but the heart rate here is 250, which is higher than typical junctional rates.

Wait, the most common SVT in infants is AVRT, specifically the orthodromic type, which uses an accessory pathway (like in WPW) and the AV node. In this case, the reentry circuit would cause a regular tachycardia with a narrow QRS. The absence of P waves could be because the atrial activation is happening via the accessory pathway, leading to a retrograde P wave that's not visible. Alternatively, if the tachycardia is AVNRT, there might be a hidden P wave. But the key here is the age and the ECG findings.

In infants, AVRT (WPW) is a common cause of SVT. The QRS is narrow because the impulse goes through the normal pathway. The lack of P waves is because the atria are depolarized retrogradely via the accessory pathway. So the diagnosis would be AVRT. Another possibility is sinus node reentrant tachycardia, but that's less common.

Looking at the options, if the correct answer is AVRT (WPW), then the other options would be less likely. For example, sinus tach would have P waves, and the rate is lower. Atrial flutter would have flutter waves. Ventricular tachycardia would have a wide QRS. So the correct answer is AVRT.

**Core Concept**
This question assesses recognition of supraventricular tachycardia (SVT) subtypes in infants. Key ECG features include narrow QRS (220 bpm in infants), pointing to reentrant arrhythmias like **atrioventricular reentrant tachycardia (AVRT)**.

**Why the Correct Answer is Right**
The clinical scenario aligns with **AVRT (Wolff-P

✓ Correct Answer: A. SVT