## **Core Concept**
Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells (hemolysis), bone marrow failure, and the presence of blood clots (thrombosis). It results from mutations in a specific gene that is crucial for the synthesis of certain proteins on the surface of blood cells.

## **Why the Correct Answer is Right**
The correct answer, **PIGA (Phosphatidylinositol Glycan class A) gene**, is the gene responsible for PNH. The PIGA gene provides instructions for making a protein that is involved in the first step of glycosylphosphatidylinositol (GPI) anchor synthesis. GPI anchors are crucial for attaching many types of proteins to the cell membrane. Mutations in the PIGA gene lead to a deficiency in GPI anchors on the surface of hematopoietic stem cells, which results in the inability of these cells to produce certain surface proteins, notably CD55 and CD59. CD55 (decay-accelerating factor) and CD59 (membrane inhibitor of reactive lysis) protect red blood cells from complement-mediated lysis. Without these protective proteins, red blood cells are more susceptible to destruction by the complement system, leading to hemolysis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **CD55** and **CD59** are indeed related to PNH, as they are the proteins whose deficiency leads to the clinical manifestations of the disease, they are not the genes in which mutations occur. Instead, they are the downstream effectors whose lack is a result of the PIGA mutation.
- **Option B:** **Factor H** is a complement regulator that inhibits the alternative pathway of complement activation. While it's involved in the complement system, mutations in Factor H are associated with atypical hemolytic uremic syndrome (aHUS), not PNH.
- **Option C:** There seems to be no commonly recognized gene or protein directly associated with PNH by this designation in standard medical literature.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for PNH is that it often presents with a triad of hemolytic anemia, pancytopenia (due to bone marrow failure), and thrombosis. The diagnosis can be confirmed by flow cytometry demonstrating a deficiency of CD55 and CD59 on the surface of blood cells.

## **Correct Answer:** . **PIGA**