**Core Concept**
Beta-2 microglobulin (β2M) is a small, non-glycosylated protein that serves as a component of the MHC class I molecule on the surface of almost all nucleated cells. Elevated levels of β2M in the serum have been associated with the progression and prognosis of certain diseases.

**Why the Correct Answer is Right**
Beta-2 microglobulin is a marker for renal function and is also used as a prognostic marker in multiple myeloma. In multiple myeloma, β2M levels are elevated due to the increased cell turnover and release of β2M from the myeloma cells. Elevated β2M levels have been correlated with disease progression, treatment response, and survival in multiple myeloma patients. The use of β2M as a prognostic marker in multiple myeloma has been well established, and it is included in the International Staging System (ISS) for multiple myeloma.

**Why Each Wrong Option is Incorrect**
**Option A:** Mycosis fungoides is a type of cutaneous T-cell lymphoma, and while β2M levels may be elevated in some cases, it is not a specific marker for this disease. Other markers such as Sézary cells and CD4/CD8 ratios are more specific for mycosis fungoides.
**Option B:** B-Cell lymphoma is a broad category of lymphomas, and β2M levels may be elevated in some cases, but it is not a specific marker for this disease. Other markers such as CD20 and CD19 are more specific for B-Cell lymphomas.
**Option D:** Mantle cell lymphoma is a type of B-Cell lymphoma, and while β2M levels may be elevated in some cases, it is not a specific marker for this disease. Other markers such as cyclin D1 and SOX11 are more specific for mantle cell lymphoma.

**Clinical Pearl / High-Yield Fact**
Elevated β2M levels are associated with a worse prognosis in multiple myeloma, and monitoring β2M levels can help guide treatment decisions. A threshold of 3.5 mg/L or higher is often used to indicate a high-risk disease.

**✓ Correct Answer: A. Multiple Myeloma**