An 8–month–old female child presented to emergency with a heart rate of 220/minute and features of congestive heart failure. Her heart rate comes down to normal after administering intravenous adenosine. What is the most likely diagnosis ?

**Core Concept**
The underlying principle being tested is the diagnosis and management of supraventricular tachycardia (SVT) in pediatric patients, specifically the use of **adenosine** as a diagnostic and therapeutic agent. **Adenosine** works by transiently blocking **AV node** conduction, thereby interrupting the re-entrant circuit that maintains SVT.

**Why the Correct Answer is Right**
The correct answer is related to the effectiveness of **adenosine** in converting SVT to a normal sinus rhythm, which suggests that the arrhythmia is likely due to a re-entrant mechanism involving the **AV node**. The fact that the heart rate returns to normal after **adenosine** administration supports this diagnosis. **Adenosine** is highly effective in terminating SVT, particularly in pediatric patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the type of arrhythmia that would respond to **adenosine**.
**Option B:** Incorrect as it is not directly related to the use of **adenosine** in diagnosing and treating arrhythmias.
**Option C:** Incorrect because, although it might be a type of arrhythmia, it is not the most likely diagnosis given the response to **adenosine**.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **adenosine** is the first-line treatment for SVT in pediatric patients due to its efficacy and safety profile. Its rapid onset and short half-life make it ideal for diagnosing and treating SVT in emergency settings.

**Correct Answer:** D. Supraventricular Tachycardia (SVT)

✓ Correct Answer: C. Paroxysmal supraventricular tachycardia