**Core Concept**
Multiple myeloma is a type of plasma cell dyscrasia characterized by the proliferation of malignant plasma cells in the bone marrow, leading to various clinical manifestations. The diagnosis of multiple myeloma involves a combination of minor and major diagnostic criteria. Minor criteria include laboratory findings and imaging studies that support the diagnosis.

**Why the Correct Answer is Right**
Plasmacytoma on biopsy is actually a major diagnostic criterion for multiple myeloma, not a minor criterion. This is because the presence of a plasmacytoma, which is a discrete mass of malignant plasma cells, is a definitive marker of the disease. A biopsy of the plasmacytoma can provide a definitive diagnosis of multiple myeloma. In contrast, minor criteria include laboratory findings such as anemia, hypercalcemia, and renal impairment, as well as imaging studies like lytic bone lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** Lytic bone lesions are a common finding in multiple myeloma, resulting from the destruction of bone by the malignant plasma cells. They are considered a minor diagnostic criterion.

**Option B:** Plasmacytosis greater than 20% is a minor diagnostic criterion for multiple myeloma. This refers to the percentage of bone marrow cells that are plasma cells, and a value greater than 20% is indicative of the disease.

**Option D:** A monoclonal globulin spike on serum electrophoresis of > 2.5 g/dl for IgG, >1.5 g/dl for IgA is a minor diagnostic criterion for multiple myeloma. This refers to the presence of a monoclonal protein in the serum, which is a hallmark of the disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the diagnosis of multiple myeloma involves a combination of minor and major diagnostic criteria. While minor criteria can support the diagnosis, a major criterion like plasmacytoma on biopsy is required for a definitive diagnosis.

**✓ Correct Answer: C. Plasmacytoma on biopsy**