**Question:** The antigen lacking in Rh negative person is -

A. Rh antigen
B. Kell antigen
C. MNSs antigen
D. Duffy antigen

**Core Concept:**
Rh blood group system is a complex of antigens and antibodies that define the Rh factor, a protein found on the surface of red blood cells (RBCs). Rh can be either positive or negative, with Rh positive being the most common. The Rh negative individuals lack specific antigens and antibodies associated with the Rh factor.

**Why the Correct Answer is Right:**
Individuals with Rh negative blood type lack the D antigen (Duffy antigen), which is responsible for recognizing the Duffy antigen on the surface of RBCs. This absence of the D antigen in Rh negative individuals results in their inability to form antibodies against the D antigen.

**Why Each Wrong Option is Incorrect:**
A. Option A (Rh antigen) is incorrect because Rh negative individuals possess the Rh antigen, but lack the D antigen (Duffy antigen).
B. Option B (Kell antigen) is incorrect as it is unrelated to the Rh blood group system and is part of the Kell blood group system.
C. Option C (MNSs antigen) is incorrect as it is part of the MNSs blood group system, not related to the Rh blood group system.
D. Option D (Duffy antigen) is incorrect because it is the antigen lacking in Rh negative individuals, not part of the Rh blood group system.

**Clinical Pearl:**
Determining an individual's blood type is crucial for safe blood transfusions and preventing transfusion reactions. In Rh negative individuals, the lack of Duffy antigen may lead to a condition called "Fybroid" or "Fybroid syndrome," which can result in severe anemia in patients receiving Rh-positive blood products. This highlights the importance of recognizing and choosing compatible blood products for patients based on their blood type.