**Core Concept**
Muscular dystrophy is a group of genetic disorders characterized by progressive muscle weakness and degeneration due to mutations in genes encoding muscle-associated proteins. The child's symptoms, including muscle weakness, easy fatigability, and calf muscle hypertrophy, are classic for Duchenne muscular dystrophy (DMD), an X-linked recessive disorder caused by mutations in the dystrophin gene.

**Why the Correct Answer is Right**
Duchenne muscular dystrophy is caused by mutations in the dystrophin gene (DMD), leading to a deficiency of the dystrophin protein. Dystrophin is a cytoskeletal protein that plays a crucial role in maintaining muscle cell integrity and structure. The dystrophin gene is a large gene, spanning over 2.4 million base pairs, and its primary transcript undergoes extensive splicing to produce a mature mRNA. Mutations in the dystrophin gene can lead to aberrant splicing, resulting in a truncated or absent dystrophin protein. This leads to muscle cell necrosis, inflammation, and progressive muscle weakness.

**Why Each Wrong Option is Incorrect**
**Option A:** Myosin is a protein involved in muscle contraction, but its deficiency is associated with myopathies such as myotonic dystrophy, not DMD.

**Option B:** Actin is another protein involved in muscle contraction, but its deficiency is not associated with DMD.

**Option C:** Troponin is a protein involved in muscle contraction, but its deficiency is associated with cardiomyopathies, not DMD.

**Clinical Pearl / High-Yield Fact**
Duchenne muscular dystrophy is caused by mutations in the dystrophin gene, leading to a deficiency of the dystrophin protein. The disease is characterized by progressive muscle weakness, calf muscle hypertrophy, and elevated serum levels of creatine kinase. Early diagnosis and treatment with corticosteroids can slow disease progression.

**Correct Answer: C. Dystrophin**