Your patient is a seven-month-old child whose mother called because he suddenly became lethargic and pale. Your assessment reveals that he is in a supraventricular tachycardia at a rate of 244 per minute. The patient's extremities are mottled and his capillary refill time is 4 seconds. Which of the following is the best treatment plan for this patient?

A) Adenosine, 0.1 mg/kg rapid IV push, repeated at 0.2 mg/kg if unsuccessful
B) Verapamil, 0.3 mg/kg over 3 to 5 minutes
C) Diltiazem, 0.25 mg/kg over 2 minutes
D) Synchronized cardioversion beginning at 0.5–1.0 joules/kg

D