**Core Concept**
Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, bone marrow failure, and the potential for blood clot formation. It results from mutations in a gene that encodes a protein involved in the regulation of the complement system.

**Why the Correct Answer is Right**
The correct answer is **C. PIGA gene**. PNH is caused by a mutation in the PIGA (Phosphatidylinositol Glycan A) gene, which is responsible for the synthesis of glycosylphosphatidylinositol (GPI), a molecule that anchors proteins to the cell membrane. In PNH, the mutated PIGA gene leads to the deficiency of GPI-anchored proteins, including CD55 and CD59, which normally protect red blood cells from complement-mediated lysis.

**Why Each Wrong Option is Incorrect**
**Option A:** The VHL gene mutation is associated with von Hippel-Lindau disease, a genetic disorder that predisposes individuals to develop tumors in various organs.

**Option B:** The HFE gene mutation is linked to hereditary hemochromatosis, a condition characterized by excessive iron accumulation in the body.

**Option D:** The TTR gene mutation is associated with transthyretin amyloidosis, a rare genetic disorder that causes the accumulation of abnormal proteins in the body.

**Clinical Pearl / High-Yield Fact**
PNH is often diagnosed by flow cytometry, which detects the absence of GPI-anchored proteins on the surface of red blood cells. This test is essential for distinguishing PNH from other causes of hemolytic anemia.

**Correct Answer:** C. PIGA gene.