**Core Concept:** Amyloidosis is a group of disorders characterized by the deposition of insoluble fibrillar proteins in tissues and organs, leading to organ dysfunction. Transthyretin (previously known as serum retinol binding protein, or PBEF, or pre-albumin) is a protein synthesized primarily in the liver and choroid plexus of the brain. It transports thyroxine (T4) and retinol (vitamin A) in the bloodstream. Transthyretin is a homotetramer composed of four identical subunits, each containing 131 amino acids.

**Why the Correct Answer is Right:** The correct answer, Familial Amyloid Polyneuropathy (FAP), is a type of hereditary amyloidosis caused by a mutation in the transthyretin gene. This gene is located on chromosome 18 and codes for transthyretin protein. When the gene is mutated, the protein misfolds, forms amyloid fibrils, and is deposited in various tissues, particularly the peripheral nerves, leading to a progressive neuropathy.

**Why Each Wrong Option is Incorrect:**

A. Amyloidosis caused by transthyretin mutations is not specifically called "type A" amyloidosis. FAP is the correct term for this type of amyloidosis.

B. The correct answer does not refer to "primary" amyloidosis, which is a different type of amyloidosis characterized by monoclonal immunoglobulin light chains.

C. This option incorrectly refers to "cardiac amyloidosis," which is a different type of amyloidosis involving the heart.

D. The correct answer is familial amyloid polyneuropathy, while this option describes "AL amyloidosis," a type of amyloidosis caused by monoclonal light chains produced by plasma cells.

**Clinical Pearl:** Familial amyloid polyneuropathy is an inheritable disorder, often diagnosed in middle-aged individuals. The severity and progression of the neuropathy depend on the type of transthyretin mutation and the extent of amyloid deposition. Early diagnosis and treatment are crucial to prevent organ dysfunction and improve prognosis.