**Question:** A 5-yr old boy is diagnosed with Duchenne muscular dystrophy. There is a mutation in the promoter site for the dystrophin gene. The true statement is:

A. The dystrophin gene is not affected in Duchenne muscular dystrophy.
B. The mutation is in the coding region of the dystrophin gene.
C. The mutation leads to reduced dystrophin protein production.
D. The mutation affects the enzyme responsible for dystrophin synthesis.

**Correct Answer:**

**Core Concept:**
Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by a deficiency of dystrophin protein, which is a crucial component of the dystrophin-glycoprotein complex (DGC) that stabilizes the muscle cell membrane during contraction. Mutations in the dystrophin gene lead to reduced or absent dystrophin protein production, resulting in muscle weakness, degeneration, and replacement by adipose and fibrous tissue. In DMD, the mutation is typically in the coding region of the dystrophin gene, leading to an abnormal protein that is not fully functional or not produced at all.

**Why the Correct Answer is Right:**

The correct answer is C: The mutation leads to reduced dystrophin protein production. In Duchenne muscular dystrophy, the genetic defect is in the dystrophin gene. The mutation occurs in the coding region of the gene, altering the structure of the dystrophin protein. As a result, the produced protein is either nonfunctional or not produced at all. This leads to the deficiency of dystrophin protein in DMD, causing the characteristic features of this disorder.

**Why Each Wrong Option is Incorrect:**

A. The wrong answer claims the dystrophin gene is not involved in Duchenne muscular dystrophy. However, the genetic defect is indeed in the dystrophin gene, and mutations in the coding region lead to the deficiency of dystrophin protein.

B. This answer incorrectly mentions the mutation affecting the enzyme responsible for dystrophin synthesis. In Duchenne muscular dystrophy, the genetic defect lies in the dystrophin gene, and the mutation affects the coding region, not an enzyme.

D. This answer incorrectly suggests that the mutation affects the enzyme involved in dystrophin synthesis. The genetic defect in Duchenne muscular dystrophy is in the dystrophin gene, and mutations in the coding region result in a nonfunctional dystrophin protein or no protein at all.

**Clinical Pearls:**

Understanding the genetic basis of Duchenne muscular dystrophy is crucial for accurate diagnosis and management of patients. The disease is caused by mutations in the dystrophin gene, and mutations in the coding region lead to a nonfunctional dystrophin protein or no protein at all. This results in the deficiency of dystrophin protein, causing the characteristic features of Duchenne muscular dystrophy.