**Core Concept**
Duchenne muscular dystrophy (DMD) is a severe, X-linked recessive genetic disorder caused by mutations in a gene responsible for encoding a protein essential for muscle function. This protein, dystrophin, helps maintain muscle integrity and stability by connecting the cytoskeleton to the extracellular matrix.

**Why the Correct Answer is Right**
The correct answer is the gene responsible for encoding dystrophin. DMD is caused by mutations in the dystrophin gene (DMD), which leads to a deficiency or absence of dystrophin protein. This results in progressive muscle degeneration and weakness. The dystrophin gene is located on the X chromosome, explaining the X-linked recessive inheritance pattern.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because it refers to the gene for Becker muscular dystrophy (BMD), a milder form of muscular dystrophy caused by mutations in the dystrophin gene that lead to a partially functional dystrophin protein.
* **Option B:** is incorrect because it refers to the gene for cystic fibrosis, a different genetic disorder caused by mutations in the CFTR gene.
* **Option C:** is incorrect because it refers to the gene for Huntington's disease, a neurodegenerative disorder caused by mutations in the Huntingtin gene.

**Clinical Pearl / High-Yield Fact**
Duchenne muscular dystrophy is characterized by progressive muscle weakness, particularly in the lower limbs, and is often diagnosed in early childhood. Early recognition and management, including physical therapy and corticosteroids, can help slow disease progression.

**Correct Answer:** D. DMD (Dystrophin Gene)