**Question:** Coomb's positive hemolysis is seen in -

A. Dengue fever
B. Hemolytic disease of the newborn (HDN)
C. Anemia due to iron deficiency
D. Autoimmune disorders

**Core Concept:**
Coomb's test is a blood test that detects the presence of antibodies (autoantibodies) against red blood cells (RBCs) in the patient's serum. This test is named after its discoverer, Dr. Thomas Coombes. Coomb's positive hemolysis is a type of hemolysis where the antibodies present in the patient's serum bind to the RBC surface, causing destruction of the RBCs via the complement system.

**Why the Correct Answer is Right:**
Coomb's positive hemolysis is mainly seen in autoimmune disorders where the immune system produces autoantibodies directed against RBC antigens. These autoantibodies bind to the RBC surface, activating the complement system and leading to destruction of the RBCs. In D, this fits perfectly as autoimmune disorders can cause such hemolysis.

**Why Each Wrong Option is Incorrect:**
A. Dengue fever: Although dengue fever can cause hemolysis, it is not due to autoantibodies attacking RBCs. It is primarily caused by the direct destruction of RBCs by the virus.
B. Hemolytic disease of the newborn (HDN): HDN is caused by antibodies present in the mother's serum that cross-react with the fetal RBC antigens, leading to hemolysis. It is not due to autoantibodies.
C. Anemia due to iron deficiency: Iron deficiency anemia is caused by insufficient RBC production, not due to autoantibodies attacking RBCs.

**Clinical Pearl:**
Coomb's test is crucial in diagnosing autoimmune hemolytic anemia and guiding the appropriate treatment. In autoimmune hemolytic anemia, the patient's serum contains autoantibodies (anti-erythrocyte antibodies) that bind to the RBCs, causing destruction of the RBCs. Treatment options depend on the underlying cause, which can be identified through Coomb's test results.

In conclusion, Coomb's positive hemolysis is primarily seen in autoimmune disorders like autoimmune hemolytic anemia, paroxysmal nocturnal hemoglobinuria (PNH), and some cases of hemolytic uremic syndrome (HUS).