**Core Concept:** Amyloidosis is a group of diseases characterized by the extracellular deposition of amyloid fibrils, which are insoluble, beta-sheet-rich fibrils, in tissues and organs. This leads to organ dysfunction and failure.

**Why the Correct Answer is Right:** The correct answer, **D.** Serum protein electrophoresis, is a key investigation for diagnosing amyloidosis. This test helps identify the type of amyloid protein involved, which is crucial for choosing the appropriate treatment.

Amyloidosis can be classified into two main types:
1. Primary (AL) amyloidosis: caused by monoclonal gammopathies (e.g., multiple myeloma, plasma cell dyscrasias).
2. Secondary (AA) amyloidosis: associated with chronic inflammatory diseases (e.g., rheumatoid arthritis, chronic infections like tuberculosis).

**Why Each Wrong Option is Incorrect:**
A. **Ultrasonography (USG):** Ultrasonography is a valuable imaging modality, but it is not a diagnostic test for amyloidosis. It is helpful for evaluating the involvement of organs like the liver, spleen, and kidneys.

B. **Computed Tomography (CT) scan:** CT scan is a useful imaging modality for assessing organ involvement, but it does not specifically diagnose amyloidosis. It helps to visualize the extent of organ involvement but does not confirm the presence of amyloid fibrils.

C. **Endomyocardial biopsy (EMB):** While EMB is essential for histopathological confirmation of amyloid deposition, it is invasive and carries risks. Serum protein electrophoresis is a non-invasive test that provides crucial information without the need for a biopsy.

**Clinical Pearl:** Amyloidosis can present with various symptoms depending on the organ system involved. Common manifestations include renal failure, anaemia, hypertension, and hypercalcaemia. Atypical symptoms like dementia, neuropathy, and cardiomyopathy may also be present.

**Correct Answer Explanation:** Serum protein electrophoresis is crucial for diagnosing amyloidosis because it measures the levels of monoclonal proteins (M-spots) in the blood. The presence of M-spots indicates a monoclonal gammopathy, which is a key feature in diagnosing amyloidosis.

By evaluating the electrophoretic pattern of serum proteins, electrophoresis can differentiate between primary and secondary amyloidosis. In primary amyloidosis (AL amyloidosis), there is an excess of monoclonal immunoglobulin (IgG, IgA, IgM, IgD, or IgE) in the serum, while in secondary amyloidosis (AA amyloidosis), there is no monoclonal protein detected in the serum.

Amyloid A amyloidosis (AA amyloidosis) is associated with chronic inflammation, often seen in rheumatoid arthritis, tuberculosis, and inflammatory bowel disease.

Amyloid light chain (AL amyloidosis) is associated with monoclonal gammopathies, such as multiple myeloma or plasma cell dyscrasias.

**Why the other options are