**Core Concept**
Multiple myeloma is a type of plasma cell dyscrasia characterized by the proliferation of malignant plasma cells in the bone marrow. Immunoglobulins, also known as antibodies, are produced by plasma cells and play a crucial role in the pathogenesis of multiple myeloma.

**Why the Correct Answer is Right**
In multiple myeloma, the malignant plasma cells produce a single type of immunoglobulin, known as a monoclonal protein or M-protein. The most common type of immunoglobulin increased in multiple myeloma is IgG, which accounts for approximately 50-60% of cases. IgG is produced by the malignant plasma cells and can interfere with the production of other immunoglobulins, leading to immunodeficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** IgA is less common than IgG in multiple myeloma, accounting for approximately 20% of cases.
**Option B:** IgM is more commonly associated with Waldenström's macroglobulinemia, a type of lymphoma.
**Option C:** IgD is the least common type of immunoglobulin increased in multiple myeloma, accounting for less than 1% of cases.

**Clinical Pearl / High-Yield Fact**
The presence of a monoclonal protein (M-protein) in the serum or urine is a diagnostic criterion for multiple myeloma. A serum protein electrophoresis (SPEP) or urine protein electrophoresis (UPEP) can help identify the type and amount of M-protein produced.

**Correct Answer:** C. IgD.