**Core Concept**
Mycosis fungoides is a rare form of cutaneous T-cell lymphoma characterized by the proliferation of malignant T-cells in the skin, leading to various clinical manifestations including erythematous patches, plaques, and tumors. The disease often progresses through distinct stages, with a predilection for the skin, but potential involvement of internal organs.

**Why the Correct Answer is Right**
Mycosis fungoides typically presents with skin lesions that are often erythematous, indurated, and may exhibit a characteristic "bathing trunk" distribution. The disease may progress through three stages: patch, plaque, and tumor stages, with the tumor stage often associated with a worse prognosis. Involvement of internal organs, such as lymph nodes, liver, and spleen, may occur in advanced stages, often accompanied by systemic symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Mycosis fungoides is typically associated with a good prognosis, especially in the early stages, which is incorrect as the disease can progress and become aggressive.
**Option B:** Mycosis fungoides is primarily a disease of the elderly, which is incorrect as it can occur at any age, but is more common in older adults.
**Option C:** Mycosis fungoides is characterized by the presence of large, Reed-Sternberg-like cells, which is incorrect as the disease is associated with the proliferation of malignant T-cells, not Reed-Sternberg cells.
**Option D:** Mycosis fungoides is a type of B-cell lymphoma, which is incorrect as it is a T-cell lymphoma.

**Clinical Pearl / High-Yield Fact**
Mycosis fungoides may be associated with a characteristic "seborrheic dermatitis-like" appearance in the early stages, which can make diagnosis challenging. A thorough skin examination and histopathological evaluation are essential for accurate diagnosis.

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