**Core Concept**
Muscle weakness in a child, associated with elevated creatine phosphokinase (CPK) levels, suggests a disorder of muscle cell (fiber) integrity, potentially involving the plasma membrane.

**Why the Correct Answer is Right**
The plasma membrane plays a crucial role in maintaining muscle cell integrity. A defect in the plasma membrane can lead to the leakage of muscle enzymes like CPK into the bloodstream. This is characteristic of Duchenne Muscular Dystrophy (DMD), a genetic disorder caused by mutations in the dystrophin gene. The absence or dysfunction of dystrophin leads to muscle fiber damage, resulting in increased CPK levels.

**Why Each Wrong Option is Incorrect**
* **Option A:** This choice is incorrect as it does not directly relate to the plasma membrane defect.
* **Option B:** This option is incorrect as it is a lysosomal storage disorder, which does not primarily affect muscle cell membrane integrity.
* **Option C:** This choice is incorrect as it is a mitochondrial disorder, which affects energy production within the muscle cell, not the plasma membrane.

**Clinical Pearl / High-Yield Fact**
Duchenne Muscular Dystrophy is an X-linked recessive disorder, primarily affecting males due to the absence of the dystrophin gene on the X chromosome. Early recognition and treatment with corticosteroids can slow disease progression.

**Correct Answer:** D. Dystrophin.