**Question:** Major fibril protein in Primary Amyloidosis is -

A. Immunoglobulin light chains (lambda or kappa)
B. Serum amyloid A protein (SAA)
C. Transcobalamin II
D. Clusterin

**Core Concept:** Amyloidosis is a group of diseases characterized by the extracellular deposition of insoluble fibrillar proteins, leading to organ dysfunction and failure. The two main types are primary (AL) and secondary (AA). In primary amyloidosis, a misfolded monoclonal immunoglobulin light chain (lambda or kappa) forms insoluble fibrils, leading to organ dysfunction.

**Why the Correct Answer is Right:** In primary amyloidosis, the abnormal production and aggregation of monoclonal immunoglobulin light chains (lambda or kappa) result in the formation of insoluble fibrils. These fibrils deposit in various organs, leading to organ dysfunction and failure. The primary amyloid fibril protein is, therefore, **Immunoglobulin light chains (lambda or kappa)**.

**Why Each Wrong Option is Incorrect:**

A. **Serum amyloid A protein (SAA):** SAA is the major acute-phase reactant protein that is produced in response to inflammation or infection. It is not the amyloid fibril protein in primary amyloidosis.

B. **Transcobalamin II:** Transcobalamin II is a vitamin B12-binding protein and is not the protein involved in primary amyloidosis.

C. **Transcobalamin II:** Similar to option B, transcobalamin II is not the protein associated with primary amyloidosis.

D. **Clusterin:** Clusterin is a protein involved in cellular protection, apoptosis, and tissue remodeling, not primary amyloidosis.

**Clinical Pearl:** The correct answer, immunoglobulin light chains (lambda or kappa), is crucial to understand in evaluating patients with suspected amyloidosis, as it helps in diagnosing and differentiating between primary and secondary amyloidosis, as well as amyloidosis and reactive amyloidosis.