Symptoms similar to McArdle disease but occurring after prolonged exercise point to which metabolic deficiency?

• A. MCAD deficiency
• B. CPT II deficiency
• C. Pompe disease
• D. GSD Type V

Answer: (A)

When sustained exercise shifts the muscle’s fuel from glycogen breakdown to fatty acid oxidation, problems in fatty acid metabolism become apparent. In medium-chain acyl-CoA dehydrogenase deficiency, the enzyme that oxidizes medium-chain fatty acids is defective, so during prolonged exercise (or fasting) the body cannot efficiently derive ATP from fats. This creates an energy shortfall in muscle and liver, leading to exercise intolerance and fatigue, and a tendency toward hypoglycemia with low ketone production (hypoketotic hypoglycemia). This pattern fits a McArdle-like picture because the issue emerges during extended activity, but the failing fuel source is fat, not glycogen. Other options describe different scenarios: CPT II deficiency usually causes rhabdomyolysis with myoglobinuria after long-duration exercise due to impaired fatty acid transport into mitochondria; Pompe disease presents with proximal weakness and cardiomyopathy from lysosomal storage; GSD type V is McArdle disease itself.