A 14-year-old boy presents with progressive muscle weakness. His parents noted difficulty climbing stairs and rising from the floor since age 4, using his hands to 'walk up' his thighs to reach a standing posture (positive Gowers' sign). Physical examination shows pseudohypertrophy of the calf muscles. Serum creatine kinase (CK) is elevated at 15,000 U/L. What is the genetic inheritance pattern of Duchenne muscular dystrophy?

Answer and explanation

Correct answer: A. X-linked recessive

  1. Step 1: Recognize clinical syndrome: Progressive proximal pelvic girdle weakness, calf pseudohypertrophy, positive Gowers' sign, and massive serum CK elevation characterizes Duchenne muscular dystrophy (DMD).
  2. Step 2: Identify gene: Mutations in the dystrophin gene located on the X chromosome (Xp21).
  3. Step 3: Confirm inheritance: X-linked recessive, primarily affecting males, with females acting as asymptomatic or mildly manifesting carriers.
Answer
X-linked recessive.
Why not the others: 'Autosomal dominant' disorders affect both sexes equally with vertical transmission; 'Autosomal recessive' requires mutations on both alleles; 'Mitochondrial' transmission exhibits maternal inheritance affecting all offspring.

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