A 4-year-old boy is brought for delay in walking. Examination shows proximal muscle weakness, calf pseudohypertrophy, and a positive Gowers sign. Serum creatine kinase is 24,000 U/L with a predominantly MM isoenzyme pattern. What is the most likely underlying defect?

• A Mitochondrial DNA deletion causing progressive external ophthalmoplegia
• B Autoimmune destruction of the motor end plate
• C Deficiency of acid alpha-glucosidase
• D Mutation in the dystrophin gene causing Duchenne muscular dystrophy ✓

Explanation

Massive CK-MM elevation reaching tens of thousands of units per litre in a young boy with proximal weakness, calf pseudohypertrophy, and Gowers sign is classic for Duchenne muscular dystrophy, caused by loss-of-function mutation of the X-linked dystrophin gene. Membrane instability allows continuous leak of the cytosolic MM isoform. Acid alpha-glucosidase deficiency causes Pompe disease, which features cardiomyopathy, and autoimmune end-plate disease does not produce such extreme CK release.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.