**Multiple Myeloma Investigation**

**Core Concept**
Multiple myeloma is a plasma cell malignancy characterized by the proliferation of malignant plasma cells in the bone marrow, leading to various complications such as bone destruction, renal impairment, and anemia. In the diagnostic workup of multiple myeloma, various investigations are employed to assess the extent of disease and guide management.

**Why the Correct Answer is Right**
The least useful investigation in multiple myeloma is the **24-hour urine light chain measurement**. This test measures the level of free light chains (FLC) in the urine, which can be elevated in multiple myeloma due to the excessive production of monoclonal immunoglobulin light chains. However, this test is not as sensitive or specific as other investigations, such as serum protein electrophoresis (SPEP) or serum FLC measurement, which can detect smaller amounts of monoclonal protein. Furthermore, a normal 24-hour urine light chain measurement does not rule out multiple myeloma, as some patients may not excrete detectable amounts of light chains in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum protein electrophoresis (SPEP) is a useful investigation in multiple myeloma, as it can detect the presence of monoclonal protein in the serum. This test is often used in conjunction with serum FLC measurement to diagnose and monitor multiple myeloma.

**Option B:** Bone marrow biopsy is a critical investigation in multiple myeloma, as it provides histological evidence of the disease and helps to assess the extent of marrow involvement. This test is essential for diagnosing multiple myeloma and guiding treatment.

**Option C:** Serum calcium measurement is an important investigation in multiple myeloma, as elevated serum calcium levels are common due to bone destruction. This test can help to identify patients who may require urgent treatment to manage hypercalcemia.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with suspected multiple myeloma, remember that a normal serum FLC ratio (1.65) has a high negative predictive value for excluding multiple myeloma. This can help to rule out the disease and avoid unnecessary further investigations.

**Correct Answer:** D. 24-hour urine light chain measurement.