An 8-Year-Old Boy Is Evaluated for Exercise Intolerance

An 8-year-old boy is evaluated for exercise intolerance.  The patient experiences fatigue, muscle pain, and cramps during exercise as well as severe muscle stiffness following strenuous activity.  Physical examination is unremarkable.  A forearm ischemic exercise test is performed by applying a blood pressure cuff on the patient's exercising forearm and sampling blood lactate several minutes after the exercise.  The patient's blood samples show no rise in lactate levels.  Biochemical analysis of a muscle biopsy reveals absent lactate dehydrogenase activity.  In this patient, strenuous exercise leads to inhibition of glycolysis in skeletal muscle due to intracellular depletion of which of the following substances?

A) AMP
B) Carnitine
C) Citrate
D) FADH₂
E) NAD⁺
F) Pyruvate

Correct Answer:

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