Preventive dose of anti D gamma globulin given to mother is-
**Core Concept**
The preventive dose of anti-D gamma globulin is administered to RhD-negative mothers to prevent the formation of anti-D antibodies against RhD-positive fetal red blood cells, thereby preventing hemolytic disease of the newborn.
**Why the Correct Answer is Right**
The administration of anti-D gamma globulin to RhD-negative mothers is a crucial measure to prevent hemolytic disease of the newborn. This is achieved by blocking the formation of anti-D antibodies against RhD-positive fetal red blood cells. The anti-D gamma globulin binds to the RhD antigen on the fetal red blood cells, preventing the mother's immune system from recognizing them as foreign and mounting an immune response. This is a critical measure to prevent the breakdown of fetal red blood cells and subsequent anemia.
**Why Each Wrong Option is Incorrect**
**Option A:** 250 units - This is the dose of anti-D gamma globulin given to RhD-negative mothers after delivery or abortion of an RhD-positive fetus, not as a preventive dose.
**Option B:** 100 units - This is a lower dose of anti-D gamma globulin, typically given to RhD-negative mothers who have had a previous RhD-positive fetus and are at high risk of sensitization.
**Option C:** 500 units - This is an excessively high dose of anti-D gamma globulin and not the standard preventive dose.
**Clinical Pearl / High-Yield Fact**
The preventive dose of anti-D gamma globulin should be administered to RhD-negative mothers at 28 weeks of gestation and again at delivery, regardless of the fetal blood type.
**Correct Answer: D. 300 units**