A 30-year-old male presented to the OPD with complaints of dark-coloured urine for 1 week. The episodes usually occur in the night. No splenomegaly was observed on examination. Lab findings revealed, Increased indirect bilirubin Hemoglobinemia and hemoglobinuria Absent haptoglobin Raised LDH Hemosiderinuria Reticulocytosis Flow cytometry studies were done The drug which has been approved for the above condition works by blocking which component of the complement system: –

**Core Concept**
The underlying condition being tested is likely Paroxysmal Nocturnal Hemoglobinuria (PNH), a rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells (hemolysis), bone marrow failure, and the potential for blood clot formation (thrombosis). **Complement system** dysregulation plays a key role in the pathogenesis of PNH.

**Why the Correct Answer is Right**
In PNH, the **complement system** is activated, leading to the lysis of red blood cells. The drug approved for this condition, Eculizumab, works by blocking the **C5 component** of the complement system, thereby preventing the formation of the membrane attack complex and reducing hemolysis. This mechanism is crucial for managing the symptoms of PNH.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while C3 is a crucial component of the complement system, the drug specifically targets C5.
**Option B:** Incorrect as C3 is not the direct target of the drug used to treat PNH.
**Option D:** Incorrect because factor D is involved in the alternative pathway of the complement system, but it's not the target of the drug used for PNH treatment.

**Clinical Pearl / High-Yield Fact**
PNH is a condition where understanding the role of the **complement system** is crucial. The disease often presents with episodes of hemoglobinuria, especially at night, due to the lysis of red blood cells.

**Correct Answer:** D. C5