**Core Concept:** Rh typing is a blood grouping system that determines the presence or absence of the Rh antigen on red blood cells. The Rh factor is important in transfusion medicine and obstetrics, as it can influence compatibility between donors and recipients, as well as the risk of Rh-negative mothers giving birth to Rh-positive babies.

**Why the Correct Answer is Right:** The correct answer is D, because it specifically mentions the presence of D antigen, which is the key component of the Rh blood group system. The D antigen is the one that determines an individual's Rh status (positive or negative). In routine Rh typing, we also test for the other common antigens (C, c, E, e) to determine the Rh phenotype. Testing for D antigen is crucial since it influences the need for Rh immunoglobulin prophylaxis and the potential for Rh isoimmunization in pregnant women.

**Why Each Wrong Option is Incorrect:** A, B, and C are incorrect because they are not the main focus of routine Rh typing. Antigen A, B, and the presence of both A and B antigens (AB) are not as critical as the D antigen. Antigen C and its absence (C-) and E and its absence (E-) are less common antigens and do not significantly impact medical decisions as D antigen does.

**Clinical Pearl:** Understanding Rh typing is essential for safe transfusion practices and preventing complications in pregnant women. If a Rh-negative individual receives Rh-positive red blood cells, they can develop Rh antibodies, which can lead to Rh immunization and hemolytic disease of the fetus and neonate (HDFN) in pregnant women. In contrast, Rh-positive individuals have no risk of developing Rh antibodies upon receiving Rh-negative blood.

**Correct Answer:** D.