**Core Concept**
ABO incompatibility refers to the immune response against ABO blood group antigens, which are present on the surface of red blood cells. The diagnosis of ABO incompatibility relies on detecting these antigens in various bodily fluids.

**Why the Correct Answer is Right**
The detection of ABO blood group antigens is crucial in diagnosing incompatibility. The antigens can be found in various bodily fluids, including **sweat**, **saliva**, and **semen**, as these fluids come into contact with red blood cells throughout the body. **Semen**, in particular, is a rich source of ABO antigens, making it a useful specimen for ABO typing. **CSF (Cerebrospinal Fluid)**, on the other hand, is a sterile fluid that circulates within the central nervous system and does not come into contact with red blood cells, making it an unlikely source of ABO antigens.

**Why Each Wrong Option is Incorrect**
**Option A:** Sweat can indeed contain ABO antigens, making it a potential source for diagnosis.
**Option B:** Saliva is another bodily fluid that can contain ABO antigens, making it a useful specimen for ABO typing.
**Option C:** Semen is a rich source of ABO antigens, making it a useful specimen for ABO typing.

**Clinical Pearl / High-Yield Fact**
When performing ABO typing, it's essential to use the correct bodily fluid specimen to avoid false results. In general, **blood**, **sweat**, **saliva**, and **semen** are all acceptable specimens for ABO typing, while **CSF** is not.

**✓ Correct Answer: D. CSF**