**Question:** Mycosis fungoides mainly affects

A. Skin
B. Nervous system
C. Lymphatic system
D. Gastrointestinal tract

**Core Concept:** Mycosis fungoides is a type of lymphoma, a group of cancers that affect white blood cells, specifically lymphocytes. It primarily involves the skin and is part of the spectrum of diseases known as Sézary syndrome.

**Why the Correct Answer is Right:** Mycosis fungoides mainly affects the skin because it is an epidermotropic lymphoma, meaning it targets the skin first. It typically starts as a red, scaly rash (erythema elevatum diutinum) and progresses to more advanced stages like patch, plaque, nodule, and tumor stages. The skin involvement results from the malignant lymphocytes infiltrating the skin and disrupting the normal immune response.

**Why Each Wrong Option is Incorrect:**

A. Skin (Mycosis fungoides affects skin, making this option incorrect)
B. Nervous system (Mycosis fungoides does not primarily involve the nervous system, making this option incorrect)
C. Lymphatic system (While lymphoma involves the lymphatic system, the correct answer, C, is the primary site of involvement, not the primary site of onset)
D. Gastrointestinal tract (Mycosis fungoides primarily affects the skin, making this option incorrect)

**Clinical Pearl:** Mycosis fungoides is a type of cutaneous T-cell lymphoma (CTCL), a group of lymphomas characterized by the proliferation of abnormal T-cells in the skin. The disease is typically slow-growing and may be indolent, but can progress to become more aggressive.

**Correct Answer:** C. Lymphatic system

Mycosis fungoides is a type of cutaneous T-cell lymphoma (CTCL), which is a group of lymphomas characterized by the proliferation of abnormal T-cells in the skin. CTCLs are classified into four stages (MF-I to MF-IV) based on the extent of involvement. The correct answer, C (lymphatic system), is the primary site of involvement in mycosis fungoides. While the disease does involve the skin, the lymphatic system is the primary site of onset and progression.