A 5-year-old boy has progressive proximal muscle weakness, Gowers sign, and markedly elevated serum creatine kinase (15,000 U/L). Muscle biopsy shows absent dystrophin on immunohistochemistry. Dystrophin is normally located at the sarcolemma. Which statement about the dystrophin gene is correct?

• A Dystrophin is a voltage-gated ion channel protein
• B It is located on the short arm of the X chromosome at Xp22
• C Mutations in dystrophin cause limb-girdle muscular dystrophy
• D It is the largest known human gene, spanning approximately 2.4 million base pairs ✓

Explanation

The DMD gene encoding dystrophin is the largest known human gene at 2.4 million base pairs with 79 exons, explaining its high spontaneous mutation rate. It is located at Xp21 (not Xp22), causing Duchenne/Becker muscular dystrophy, not limb-girdle muscular dystrophy. Dystrophin is a cytoskeletal protein linking the actin cytoskeleton to the extracellular matrix via the dystrophin-associated glycoprotein complex, not an ion channel.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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