**Core Concept**
Multiple myeloma is a clonal plasma cell disorder characterized by the proliferation of malignant plasma cells in the bone marrow, leading to bone destruction and organ dysfunction. The diagnosis of multiple myeloma involves a combination of clinical evaluation, laboratory tests, and imaging studies to confirm the presence of malignant plasma cells and assess the extent of disease.

**Why the Correct Answer is Right**
The diagnosis of multiple myeloma typically involves a comprehensive evaluation, including serum protein electrophoresis (SPEP) to detect monoclonal immunoglobulin (M-protein), 24-hour urine protein electrophoresis (UPEP) to detect light chains, and bone marrow biopsy to confirm the presence of malignant plasma cells. **Serum free light chain (FLC) assay** is also a sensitive test for detecting light chain production. The combination of these tests helps to establish the diagnosis and assess the extent of disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum creatinine and liver function tests may be abnormal in multiple myeloma due to renal impairment and amyloidosis, but they are not diagnostic tests for the disease.
**Option B:** Bone scans may show areas of increased uptake in multiple myeloma, but they are not specific for the disease and may be positive in other conditions such as osteoporosis.
**Option C:** Peripheral blood smears may show plasma cells, but the diagnosis of multiple myeloma requires a bone marrow biopsy to confirm the presence of malignant plasma cells.

**Clinical Pearl / High-Yield Fact**
In patients with suspected multiple myeloma, a **serum free light chain (FLC) ratio** of >100:1 is highly suggestive of the disease, and a ratio of <10:1 is unlikely to be associated with multiple myeloma. **Correct Answer:** C. Bone marrow biopsy to confirm the presence of malignant plasma cells.