## **Core Concept**
The core concept being tested here involves understanding the risk of Rh incompatibility between a mother and her fetus, which can lead to hemolytic disease of the newborn (HDN), also known as erythroblastosis fetalis. This condition occurs when an Rh-negative mother develops antibodies against Rh-positive blood cells, which can then cross the placenta and destroy the fetus's red blood cells if the fetus is Rh-positive.

## **Why the Correct Answer is Right**
The correct answer, **C. Indirect Coombs test**, is the test of choice for detecting the presence of circulating anti-Rh antibodies in the mother. The Indirect Coombs test, also known as the indirect antiglobulin test, detects antibodies or complement proteins that are bound to the surface of red blood cells or present free in the serum. It is used to identify antibodies against red blood cell antigens, such as the Rh factor, in the serum of an individual. This test is crucial in the context of Rh incompatibility and potential hemolytic disease of the newborn.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The Direct Coombs test (or direct antiglobulin test) is used to detect antibodies or complement proteins that are already bound to the surface of red blood cells. It is not the best choice for detecting free-floating anti-Rh antibodies in the mother's serum.
- **Option B:** While the blood type and Rh factor determination are essential in assessing the risk of Rh incompatibility, they do not directly test for the presence of anti-Rh antibodies.
- **Option D:** The Kleihauer-Betke test is used to detect fetal-maternal hemorrhage by identifying fetal red blood cells in the maternal circulation. It does not directly test for anti-Rh antibodies.

## **Clinical Pearl / High-Yield Fact**
A critical clinical pearl is that if an Rh-negative mother is not sensitized (does not have anti-Rh antibodies), she will typically receive Rh immune globulin (RhIg) at 28 weeks of gestation and within 72 hours of delivery if the baby is Rh-positive. This prophylaxis prevents the formation of anti-Rh antibodies. However, in cases where sensitization has already occurred, as in this patient, monitoring for the presence and levels of anti-Rh antibodies is crucial to manage the risk of hemolytic disease of the newborn in future pregnancies.

## **Correct Answer:** C. Indirect Coombs test.