**Core Concept**
Monoclonal gammopathy of undetermined significance (MGUS) is a condition characterized by the presence of a monoclonal protein spike in the serum, typically IgG or IgM, without any evidence of end-organ damage or symptoms. It is often detected incidentally during routine laboratory tests.

**Why the Correct Answer is Right**
The presence of a monoclonal spike on serum electrophoresis and 8% plasma cells in the bone marrow are key diagnostic criteria for MGUS. The low percentage of plasma cells in the bone marrow and the absence of end-organ damage or symptoms support this diagnosis. The IgG level of 1.5 g/dl is also within the normal range, which further suggests MGUS. The lack of symptoms and end-organ damage in this patient makes a diagnosis of multiple myeloma or other plasma cell dyscrasias less likely.

**Why Each Wrong Option is Incorrect**
**Option A:** Multiple myeloma is a plasma cell dyscrasia characterized by >10% plasma cells in the bone marrow and end-organ damage, which is not present in this case.
**Option B:** Waldenström macroglobulinemia is a lymphoplasmacytic lymphoma characterized by high levels of IgM, which is not indicated in this case.
**Option C:** Primary amyloidosis is a condition characterized by the deposition of amyloid protein in various organs, which is not suggested by the provided information.

**Clinical Pearl / High-Yield Fact**
MGUS is a common condition in the elderly and is often incidentally detected during routine laboratory tests. Patients with MGUS require regular monitoring to assess for progression to a plasma cell dyscrasia or other complications.

**Correct Answer: C.**