An elderly male presents with headache, recurrent infections and multiple punched out lytic lesions of x-ray skull. Which of the following investigation will best help in establishing the diagnosis?
**Core Concept**
The patient's presentation suggests a diagnosis of multiple myeloma, a plasma cell malignancy characterized by the proliferation of malignant plasma cells in the bone marrow, leading to bone destruction and anemia. The clinical features of headache, recurrent infections, and multiple punched out lytic lesions on the skull are hallmarks of this disease.
**Why the Correct Answer is Right**
The diagnosis of multiple myeloma can be established by identifying clonal plasma cells in the bone marrow or by detecting monoclonal protein (M-protein) in the serum or urine. Serum protein electrophoresis (SPEP) is a sensitive test for detecting M-protein, which is a hallmark of multiple myeloma. This test separates proteins in the serum based on their size and charge, allowing for the detection of monoclonal proteins. In multiple myeloma, the M-protein is typically IgG or IgA, which can be further characterized by immunofixation electrophoresis (IFE).
**Why Each Wrong Option is Incorrect**
**Option A:** Serum calcium levels may be elevated in multiple myeloma due to bone destruction and hypercalcemia, but this is a non-specific finding and not diagnostic of the disease. While serum calcium levels may be abnormal, they do not establish the diagnosis of multiple myeloma.
**Option B:** Complete Blood Count (CBC) may show anemia, thrombocytopenia, or leukopenia in multiple myeloma, but these findings are non-specific and not diagnostic of the disease.
**Option C:** Bone marrow biopsy is a critical diagnostic test for multiple myeloma, but it is not the best initial test for establishing the diagnosis. Bone marrow biopsy can confirm the presence of clonal plasma cells, but it is typically performed after an abnormality is detected on a screening test like SPEP.
**Clinical Pearl / High-Yield Fact**
The presence of an M-protein in the serum or urine is a diagnostic hallmark of multiple myeloma. Remember the "CRAB" criteria to diagnose multiple myeloma: **C**alcium elevation, **R**enal impairment, **A**naemia, and **B**one lesions.
**Correct Answer:** C. Bone marrow biopsy is a critical diagnostic test for multiple myeloma, but it is not the best initial test for establishing the diagnosis. Bone marrow biopsy can confirm the presence of clonal plasma cells, but it is typically performed after an abnormality is detected on a screening test like SPEP.