**Core Concept**
Charcot-Marie-Tooth disease (CMT) is a group of hereditary disorders affecting the peripheral nerves, characterized by progressive muscle weakness and atrophy, primarily in the limbs. The X-linked form of CMT is a specific subtype, caused by mutations in genes responsible for encoding proteins essential for normal nerve function.

**Why the Correct Answer is Right**
The X-linked form of CMT is primarily caused by mutations in the gene encoding connexin 32 (Cx32), also known as gap junction protein alpha 1 (GJA1). This protein is crucial for maintaining the integrity and function of the myelin sheath surrounding peripheral nerve axons. Cx32 forms gap junctions with adjacent Schwann cells, facilitating the exchange of ions, metabolites, and signaling molecules necessary for proper nerve function. Mutations in the GJA1 gene disrupt this process, leading to demyelination, axonal degeneration, and the characteristic clinical features of CMTX.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the autosomal dominant form of CMT is primarily associated with mutations in the PMP22 gene, not the X-linked form.
**Option B:** This option is incorrect because the autosomal recessive form of CMT is primarily associated with mutations in the MFN2 gene, not the X-linked form.
**Option C:** This option is incorrect because the mitochondrial form of CMT is primarily associated with mutations in the MT-ATP6 and MT-ATP8 genes, not the X-linked form.

**Clinical Pearl / High-Yield Fact**
CMTX is the most common X-linked form of CMT, accounting for approximately 15-20% of all CMT cases. It is essential to consider genetic testing for the GJA1 gene in patients with a family history of CMT or those presenting with characteristic clinical features.

**Correct Answer: C. Connexin 32 (GJA1) gene mutation.**