A 6-Month-Old Boy Is Brought to the Emergency Department Due

A 6-month-old boy is brought to the emergency department due to lethargy for several hours.  His mother says he is usually a "very active baby," and she is afraid that he is "coming down with something."  The patient's birth history is unremarkable, and he was born at 38 weeks gestation.  He is afebrile.  Blood pressure is 70/44 mm Hg, pulse is 240 beats/min, and respirations are 52/min.  The patient appears pale and is perspiring.  Capillary refill time is less than 2 seconds.  The lungs are clear to auscultation.  No murmurs are heard.  An ECG shows a regular heart rate of 240/min with narrow QRS complexes and no identifiable P waves.  Which of the following is the most likely cause of this patient's hypotension?

A) Decreased effective intravascular volume
B) Decreased myocardial contractility
C) Decreased peripheral vascular resistance
D) Left-to-right intracardiac shunting
E) Shortening of ventricular diastole

Correct Answer:

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