**Core Concept**
The question is testing the diagnosis of a pediatric patient with progressive muscle weakness and walking difficulties. This clinical presentation is suggestive of a neuromuscular disorder, specifically one that affects the child's ability to walk and maintain muscle strength.

**Why the Correct Answer is Right**
The probable diagnosis is Duchenne Muscular Dystrophy (DMD), a genetic disorder caused by mutations in the dystrophin gene. DMD leads to the absence or deficiency of the dystrophin protein, which is essential for muscle function. The absence of dystrophin results in progressive muscle degeneration and weakness, particularly affecting the proximal muscles. The clinical presentation of DMD typically includes muscle weakness, wasting, and gait disturbances, which are consistent with the symptoms described in the question.

**Why Each Wrong Option is Incorrect**
* **Option A:** Spinal Muscular Atrophy (SMA) typically presents with muscle weakness and wasting, but it primarily affects the lower motor neurons and does not typically cause the same level of muscle degeneration as DMD.
* **Option B:** Muscular Dystrophy (MD) is a broad term that encompasses several genetic disorders, including DMD. However, DMD is a specific type of muscular dystrophy that is caused by mutations in the dystrophin gene.
* **Option C:** Becker Muscular Dystrophy (BMD) is a milder form of muscular dystrophy that is also caused by mutations in the dystrophin gene. BMD typically presents with a later age of onset and slower progression than DMD.

**Clinical Pearl / High-Yield Fact**
DMD is an X-linked recessive disorder, meaning that it primarily affects boys who inherit the mutated gene from their mothers. The diagnosis of DMD can be confirmed through genetic testing, which involves analyzing the dystrophin gene for mutations.

**Correct Answer:** D. Duchenne Muscular Dystrophy (DMD)