**Core Concept**
The key symptom in alcohol withdrawal syndrome is a manifestation of the body's adaptation to the absence of its primary depressant, ethanol. This adaptation involves a hyperactive response due to the upregulation of neurotransmitters, particularly GABA, and the downregulation of excitatory neurotransmitters, such as glutamate.

**Why the Correct Answer is Right**
The correct answer is **tremors**. Tremors occur due to the increased activity of the sympathetic nervous system, which stimulates the release of excitatory neurotransmitters. The upregulation of GABA receptors, in an attempt to compensate for the loss of ethanol, also contributes to tremors. This symptom is often the first sign of alcohol withdrawal and can progress to more severe symptoms, such as seizures and delirium tremens.

**Why Each Wrong Option is Incorrect**
* **Option A:** Confusion is a symptom of alcohol withdrawal, but it is not the key symptom. Confusion is a result of the complex interplay between excitatory and inhibitory neurotransmitters.
* **Option B:** Hallucinations are a symptom of delirium tremens, which is a severe form of alcohol withdrawal. While hallucinations are a key symptom, they are not the first sign of withdrawal.
* **Option C:** Seizures are a symptom of alcohol withdrawal, but they are not the key symptom. Seizures are a result of the extreme imbalance between excitatory and inhibitory neurotransmitters.

**Clinical Pearl / High-Yield Fact**
The severity of alcohol withdrawal symptoms is often classified using the Clinical Institute Withdrawal Assessment for Alcohol (CIWA-Ar) scale. This scale helps clinicians assess the severity of withdrawal and provide appropriate treatment.

**Correct Answer: C. Seizures**