**Core Concept**
The Rh factor is a specific antigen present on the surface of red blood cells, which can trigger an immune response in individuals who are Rh-negative. This antigen is crucial in transfusion medicine and pregnancy, as it can cause hemolytic disease of the newborn.

**Why the Correct Answer is Right**
The Rh factor is an antigen encoded by the RHD gene, located on the short arm of chromosome 1. It is a transmembrane protein that spans the red blood cell membrane, with the majority of its structure exposed to the external environment. The Rh antigen can trigger an immune response in individuals who are Rh-negative, leading to the production of antibodies against the Rh-positive red blood cells. This can cause hemolysis and other complications in transfusion medicine and pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable (no information provided).
**Option B:** Incorrect - The Rh factor is not an enzyme, but rather an antigen.
**Option C:** Incorrect - The Rh factor is not a hormone, but rather a protein.

**Clinical Pearl / High-Yield Fact**
The Rh factor is a crucial consideration in transfusion medicine, as Rh-positive blood can be transfused to Rh-negative individuals without significant risk. However, Rh-negative individuals should not receive Rh-positive blood, as this can trigger an immune response and lead to hemolysis.

**Correct Answer: D. Antigen**