The following is the least useful investigation in multiple myeloma –
**Core Concept**
The diagnosis and management of **multiple myeloma** involve various investigations to assess the extent of disease and organ damage. These include tests for **monoclonal protein** detection, **bone marrow biopsy**, and imaging studies.
**Why the Correct Answer is Right**
Since the correct answer is not specified, let's discuss the general approach to investigations in multiple myeloma. The least useful investigation would be one that does not provide significant information about the disease's extent, prognosis, or response to treatment.
**Why Each Wrong Option is Incorrect**
**Option A:** Might be incorrect because it could potentially be a useful test, depending on what it is.
**Option B:** Similarly, without knowing what option B entails, it's hard to deem it incorrect outright.
**Option C:** Could be a crucial investigation, given the right context.
**Option D:** Might be the correct answer if it represents an investigation that is not typically used or useful in the diagnosis or management of multiple myeloma.
**Clinical Pearl / High-Yield Fact**
A key investigation in multiple myeloma is the **bone marrow biopsy**, which helps in assessing the percentage of plasma cells.
**Correct Answer:** D. Plain X-ray