## Core Concept
The administration of prophylactic anti-D immunoglobulin is a critical measure to prevent the sensitization of Rh-negative mothers to Rh-positive blood, which can occur during childbirth if the baby is Rh-positive. This is essential to prevent hemolytic disease of the newborn (HDN) in future pregnancies.

## Why the Correct Answer is Right
The correct approach involves administering anti-D immunoglobulin to the Rh-negative mother within 72 hours of delivering an Rh-positive baby. This intervention helps to neutralize any Rh-positive fetal red blood cells that may have entered the mother's circulation during delivery, thereby preventing an immune response against Rh-positive blood. The correct timing and indication are crucial for the effectiveness of this prophylaxis.

## Why Each Wrong Option is Incorrect
- **Option A:** Immediate administration is not always necessary; the critical timeframe for administration is within 72 hours.
- **Option B:** Only within 24 hours is too narrow a window; the accepted timeframe extends up to 72 hours.
- **Option C:** After 72 hours is too late; the prophylactic effect is most beneficial when administered within the specified timeframe.
- **Option D (Correct Answer):** Within 72 hours is the correct timeframe for administering prophylactic anti-D immunoglobulin.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the administration of anti-D immunoglobulin is not only indicated postpartum but also in other scenarios such as after abortion, ectopic pregnancy, or any event that could lead to the introduction of fetal Rh-positive cells into the maternal circulation.

**Correct Answer: D. Within 72 hours.**