**Core Concept**
Mycosis fungoides is a type of cutaneous T-cell lymphoma characterized by a malignant proliferation of epidermotropic T lymphocytes. It is the most common form of primary cutaneous lymphoma, often presenting with skin lesions that mimic eczema or psoriasis.

**Why the Correct Answer is Right**
Mycosis fungoides typically involves the skin, but it can also involve other organs such as lymph nodes, liver, and spleen. The disease progresses through various stages, starting with patches and plaques, followed by tumors, and eventually, it can progress to erythroderma and extracutaneous involvement. The epidermotropic T lymphocytes in mycosis fungoides express CD4, CD7, and CD2, but often lose CD3 and CD8 expression.

**Why Each Wrong Option is Incorrect**
**Option A:** Mycosis fungoides is more common in individuals with a history of atopy. This statement is incorrect because mycosis fungoides is more common in individuals with a history of eczema, but not necessarily atopy.

**Option B:** Mycosis fungoides is usually associated with a poor prognosis. This statement is incorrect because the prognosis of mycosis fungoides varies depending on the stage and extent of disease.

**Option C:** Mycosis fungoides is a type of B-cell lymphoma. This statement is incorrect because mycosis fungoides is a type of T-cell lymphoma.

**Option D:** Mycosis fungoides is characterized by the presence of Reed-Sternberg cells. This statement is incorrect because Reed-Sternberg cells are characteristic of Hodgkin lymphoma, not mycosis fungoides.

**Clinical Pearl / High-Yield Fact**
Mycosis fungoides often presents with skin lesions that mimic eczema or psoriasis, and a skin biopsy is essential for diagnosis. Remember the "SEVEN Ps" to differentiate mycosis fungoides from eczema: Patches, Plaques, Pruritus, Peripheral scaling, Pustules, Poikiloderma, and Perforation.

**Correct Answer: C. Mycosis fungoides is a type of B-cell lymphoma.**