Paroxysmal Nocturnal Hemoglobinuria (PN ) is screened by -a) Acid ham testb) Sucrose lysis testc) Serum hapten levelsd) Low serum complement levels

**Core Concept**
Paroxysmal Nocturnal Hemoglobinuria (PNH) is a rare acquired disorder characterized by the destruction of red blood cells, leading to hemolytic anemia. The condition is caused by a deficiency of the complement regulatory protein CD55 (decay-accelerating factor) on the surface of red blood cells, making them more susceptible to complement-mediated lysis.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of PNH. The condition is associated with low levels of serum complement, particularly the complement system's terminal components (C5b-C9), which form the membrane attack complex (MAC) that causes cell lysis. In PNH, the absence of CD55 and CD59 (protectin) on the surface of red blood cells prevents the regulation of complement activation, leading to uncontrolled MAC formation and subsequent cell destruction.

**Why Each Wrong Option is Incorrect**
* **Option A:** The acid hemolysin test is used to detect the presence of antibodies against the Rh blood group system, not PNH.
* **Option B:** The sucrose lysis test is a screening test for paroxysmal nocturnal hemoglobinuria, but it is not the most specific or commonly used method. The correct answer is more directly related to the underlying pathophysiology of PNH.
* **Option C:** Serum hapten levels are not directly related to PNH. Haptens are small molecules that can bind to antibodies, but they are not typically associated with the complement system or PNH.

**Clinical Pearl / High-Yield Fact**
PNH is often associated with bone marrow failure and can progress to aplastic anemia or myelodysplastic syndromes. Patients with PNH may also have a higher risk of developing thrombosis due to the activation of the coagulation cascade.

**Correct Answer:** B. Sucrose lysis test