for progressive muscle weakness

Question

A 3-year-old boy is evaluated for progressive muscle weakness and cardiac arrhythmias. He has a history of failure to thrive. Laboratory studies show elevated creatine kinase and lipid profile reveals hypertriglyceridemia. A muscle biopsy shows abnormal accumulation of lipid droplets. Which of the following is the most likely diagnosis?

A. McArdle disease
B. Pompe disease
C. Carnitine deficiency
D. Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency
E. Von Gierke disease

Correct Answer: C. Carnitine deficiency

Explanation: This patient presents with muscle weakness, cardiomyopathy, and lipid accumulation in muscle—the classic triad of carnitine deficiency. Carnitine is essential for transporting long-chain fatty acids into the mitochondria for β-oxidation. Deficiency leads to impaired fatty acid oxidation, causing lipid accumulation in muscle (explaining weakness and elevated CK) and heart (explaining cardiomyopathy/arrhythmias).

Why other options are incorrect:

• A (McArdle disease) : Glycogen storage disease type V (muscle phosphorylase deficiency). Presents with exercise intolerance, cramps, and second wind phenomenon. Lipid accumulation is NOT a feature.
• B (Pompe disease) : Glycogen storage disease type II (acid α-1,4-glucosidase deficiency). Presents with cardiomyopathy and hypotonia, but glycogen (not lipid) accumulates in lysosomes. Infantile form is rapidly progressive.
• D (MCAD deficiency) : Presents with hypoketotic hypoglycemia during fasting, typically in infancy. Does NOT cause muscle weakness or cardiomyopathy as primary features. Lipid accumulation is not prominent in muscle.