**Core Concept**
Mycosis fungoides is a rare type of cutaneous T-cell lymphoma characterized by the malignant proliferation of T-lymphocytes in the skin. It is the most common form of primary cutaneous lymphoma, typically presenting as a rash or skin lesions.

**Why the Correct Answer is Right**
Mycosis fungoides involves the clonal expansion of malignant T-cells in the skin, leading to the characteristic skin lesions. The T-cells in mycosis fungoides are typically CD4+ helper T-cells, which are activated in response to antigens in the skin. The disease progresses through various stages, starting from patches and plaques to tumors and eventually systemic involvement. The pathogenesis of mycosis fungoides is not fully understood, but it is believed to involve genetic mutations and epigenetic alterations that lead to the uncontrolled proliferation of malignant T-cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Sézary syndrome is a leukemic variant of mycosis fungoides, characterized by the presence of malignant T-cells in the blood. While Sézary syndrome is a related condition, it is not the primary condition affected by mycosis fungoides.
**Option B:** Cutaneous T-cell lymphoma is a broader category of lymphomas that includes mycosis fungoides, but it is not the specific condition affected by mycosis fungoides.
**Option C:** Mycosis fungoides is not a type of skin cancer, but rather a type of lymphoma that involves the skin.

**Clinical Pearl / High-Yield Fact**
Mycosis fungoides often presents with non-specific skin symptoms, such as itching, redness, and scaling, which can delay diagnosis. A high index of suspicion is necessary to diagnose mycosis fungoides, especially in patients with a history of skin lesions or lymphoma.

**Correct Answer: D. Skin.**