**Core Concept**
The infant in question is likely suffering from supraventricular tachycardia (SVT), a common arrhythmia in neonates and infants characterized by a rapid heart rate originating from the atria. The heart rate of 240 bpm, which remains constant despite crying, is a hallmark of SVT.

**Why the Correct Answer is Right**
Carotid massage is a first-line treatment for SVT in infants. The vagus nerve, which runs through the carotid sheath, stimulates the sinoatrial node, leading to a decrease in heart rate. When performed correctly, carotid massage can temporarily stop the SVT, allowing the heart to return to a normal rhythm. This is because the vagus nerve stimulates the release of acetylcholine, which inhibits the sinoatrial node and slows the heart rate.

**Why Each Wrong Option is Incorrect**
**Option A:** Rapid verapamil infusion is not the first-line treatment for SVT in infants. Verapamil is a calcium channel blocker that can be used to treat SVT in adults, but its use in infants is not recommended due to the risk of severe hypotension and cardiac arrest.

**Option B:** Transthoracic pacing of the heart is not a first-line treatment for SVT in infants. Pacemakers are typically used in cases of bradycardia or heart block, not tachycardia.

**Option D:** DC cardioversion is not the first-line treatment for SVT in infants. Cardioversion involves the use of electrical shocks to convert an abnormal heart rhythm to a normal one. While it can be effective in treating SVT, it is generally reserved for cases where carotid massage is unsuccessful or not feasible.

**Clinical Pearl / High-Yield Fact**
In infants with SVT, carotid massage is often effective because the vagus nerve is still developing and is more easily stimulated. This is why carotid massage is a first-line treatment for SVT in this age group.

**✓ Correct Answer: C. Carotid massage**