**Core Concept**
Frey's syndrome, also known as gustatory sweating or auriculotemporal syndrome, is a rare neurological disorder that occurs due to aberrant regeneration of nerve fibers between the parotid gland and the overlying skin. This results in inappropriate cross-talk between the parasympathetic and sympathetic nervous systems, leading to excessive cholinergic activity in response to stimuli that normally trigger salivation.

**Why the Correct Answer is Right**
The neurotransmitter involved in Frey's syndrome is acetylcholine (ACh). Normally, ACh is released from the parasympathetic nerve terminals of the auriculotemporal nerve, which innervates the parotid gland, causing salivation. In Frey's syndrome, the aberrant regeneration of nerve fibers leads to the release of ACh in response to gustatory stimuli, causing facial flushing, sweating, and drooling.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not applicable, as it is blank.
* **Option B:** Although norepinephrine is a neurotransmitter involved in the sympathetic nervous system, it is not the primary neurotransmitter implicated in Frey's syndrome.
* **Option C:** This option is not a neurotransmitter involved in Frey's syndrome.

**Clinical Pearl / High-Yield Fact**
Frey's syndrome often occurs after surgical procedures involving the parotid gland, such as parotidectomy. It is essential to be aware of this potential complication and to take steps to prevent it during such surgeries.

**Correct Answer:** A. Acetylcholine.