**Core Concept**
Glutamine is a non-essential amino acid that plays a crucial role in various physiological processes, including nitrogen transport, energy metabolism, and gut health. In the blood, glutamine serves as a key fuel source for enterocytes, which are the cells lining the small intestine.

**Why the Correct Answer is Right**
Glutamine is indeed the primary fuel source for enterocytes, which requires a constant supply to maintain their energy needs and support intestinal barrier function. This is particularly important during times of stress, injury, or infection, when the demand for glutamine increases to support the healing process. The enzyme glutaminase breaks down glutamine into glutamate and ammonia, which are then used by enterocytes to produce energy through the citric acid cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutamine does not primarily act as a neurotransmitter in the blood. While it can be converted into neurotransmitters such as glutamate and GABA, this is not its primary function in the bloodstream.
**Option B:** Glutamine does not primarily act as an antioxidant in the blood. While it can have antioxidant properties, its main role is as a fuel source for enterocytes.
**Option C:** Glutamine does not primarily act as a precursor to purines in the blood. While it can be converted into purines, this is not its primary function in the bloodstream.

**Clinical Pearl / High-Yield Fact**
Glutamine supplementation is often used to support gut health and reduce morbidity in critically ill patients, particularly those with severe burns or traumatic injuries.

**Correct Answer: C. Fuel source for enterocytes.**