**Core Concept**
Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive muscle degeneration and weakness. It is caused by mutations in the dystrophin gene, leading to the absence or deficiency of the dystrophin protein, which is essential for muscle function.

**Why the Correct Answer is Right**
Duchenne muscular dystrophy typically presents in early childhood, with symptoms such as muscle weakness, particularly in the proximal muscles (e.g., difficulty climbing stairs or getting up from the floor). The absence of dystrophin protein disrupts the muscle cell membrane, leading to muscle damage and degeneration. The muscle biopsy findings in this case, including small muscle fibrils and absent dystrophin, are consistent with DMD. This condition is inherited in an X-linked recessive pattern, making it more common in males.

**Why Each Wrong Option is Incorrect**
**Option A:** Beck's muscle dystrophy is a rare form of muscular dystrophy that presents in adulthood, with symptoms such as muscle weakness, wasting, and contractures. It is not typically associated with the absence of dystrophin protein.
**Option C:** Myotonic dystrophy is a type of muscular dystrophy characterized by muscle stiffness (myotonia) and wasting, particularly in the distal muscles. It is caused by mutations in the DMPK gene and is not associated with the absence of dystrophin protein.
**Option D:** Limb-girdle muscular dystrophy is a group of genetic disorders characterized by muscle weakness and wasting, particularly in the proximal muscles. While it can present in childhood, it is not typically associated with the absence of dystrophin protein.

**Clinical Pearl / High-Yield Fact**
Duchenne muscular dystrophy is a classic example of a genetic disorder that can be diagnosed through genetic testing, muscle biopsy, or family history. Early recognition and treatment, such as corticosteroids and physical therapy, can help slow disease progression and improve quality of life.

**✓ Correct Answer: B. Duchenne muscular dystrophy**