Your patient is found sitting on the edge of the bathtub with cool, diaphoretic skin. She states she became lightheaded and nearly "passed out" while vomiting. Your cardiac monitor shows a sinus bradycardia at a rate of 48. Which of the following is most likely?

A) Sick sinus syndrome
B) Use of sympathomimetic medications
C) Disease of the cardiac conduction system
D) Increased parasympathetic tone

D