**Core Concept**
Multiple Myeloma is a type of blood cancer characterized by the proliferation of malignant plasma cells in the bone marrow, leading to the overproduction of a single immunoglobulin (Ig) clone. The most common types of immunoglobulins involved in Multiple Myeloma are IgG, IgA, and less frequently, IgM.

**Why the Correct Answer is Right**
The correct answer is IgD. This is because IgD is the least common immunoglobulin subtype involved in Multiple Myeloma, accounting for only a small percentage of cases. The pathophysiology of Multiple Myeloma involves the accumulation of malignant plasma cells, which can produce excessive amounts of a single Ig subtype. The rarity of IgD myeloma is thought to be due to the limited expression of the IgD heavy chain gene in normal plasma cells.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is the most common subtype involved in Multiple Myeloma, accounting for approximately 50-60% of cases. This is because IgG is the most abundant immunoglobulin in the human body and is produced by a large percentage of plasma cells.

**Option B:** IgA is the second most common subtype involved in Multiple Myeloma, accounting for approximately 20-30% of cases. IgA myeloma often presents with specific clinical features, such as renal failure and hypercalcemia.

**Option C:** IgM is a less common subtype involved in Multiple Myeloma, accounting for approximately 5-10% of cases. IgM myeloma often presents with specific clinical features, such as hyperviscosity syndrome and renal failure.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the clinical presentation of Multiple Myeloma can vary depending on the subtype of immunoglobulin involved. For example, IgG myeloma often presents with bone lesions and hypercalcemia, while IgA myeloma may present with renal failure and hypercalcemia.

**Correct Answer:** D. IgD.