## **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). The condition arises due to a mutation in the PIGA gene of hematopoietic stem cells in the bone marrow, leading to a deficiency in the surface proteins that protect red blood cells from complement-mediated lysis.

## **Why the Correct Answer is Right**
The correct answer involves the deficiency of two surface proteins: **CD55 (Decay-accelerating factor, DAF)** and **CD59 (Membrane inhibitor of reactive lysis, MIRL)**.
- **CD55** accelerates the decay of the complement C3 convertase, preventing the formation of the membrane attack complex (MAC) that can lyse cells.
- **CD59** inhibits the formation of the MAC by binding to complement components and preventing their incorporation into the membrane.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify the correct proteins.
- **Option B:** This option is incorrect because it also does not specify the correct proteins related to PNH.
- **Option C:** This option might mention one correct protein but lacks the second one, making it incomplete.
- **Option D:** This option is incorrect as it does not accurately represent the deficient proteins in PNH.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for PNH is that the disease is associated with a high risk of thrombosis, and patients often present with symptoms such as hemoglobinuria (which causes the urine to appear dark or tea-colored), fatigue, and shortness of breath due to hemolysis. The diagnosis of PNH involves flow cytometry to detect the deficiency of CD55 and CD59 on the surface of blood cells.

## **Correct Answer:** D. CD55 and CD59.