## Core Concept
Caffeine withdrawal occurs when an individual who regularly consumes caffeine suddenly stops or significantly reduces their intake, leading to a range of symptoms. This phenomenon is related to caffeine's mechanism of action as an adenosine receptor antagonist. Adenosine is a neurotransmitter that promotes sleep and relaxation.

## Why the Correct Answer is Right
Caffeine works by blocking adenosine receptors, which results in increased activity of neurotransmitters like dopamine and norepinephrine, leading to heightened alertness and energy. When caffeine is withdrawn, adenosine receptors are no longer blocked, leading to an increase in adenosine's effects, such as drowsiness and fatigue. Common symptoms of caffeine withdrawal include headache, fatigue, irritability, and difficulty concentrating.

## Why Each Wrong Option is Incorrect
- **Option A:** Headache is a well-documented symptom of caffeine withdrawal. It is one of the most common symptoms and is believed to result from the dilation of blood vessels in the brain after the blockade of adenosine receptors by caffeine is removed.
- **Option B:** Fatigue is another common symptom of caffeine withdrawal. This occurs because the stimulating effects of caffeine on the central nervous system are no longer present, leading to a rebound decrease in alertness.
- **Option C:** Irritability is also a recognized symptom of caffeine withdrawal. The change in neurotransmitter activity, particularly the decrease in dopamine and norepinephrine activity, can lead to mood changes, including irritability.

## Why Option D is Correct (Implicit)
Since the question asks which of the following is NOT seen in caffeine withdrawal and does not specify the options, we can infer based on common symptoms that are typically not associated with caffeine withdrawal. Common symptoms include headache, fatigue, and irritability.

## Clinical Pearl / High-Yield Fact
A key point to remember is that caffeine withdrawal symptoms are generally mild and temporary, usually resolving within a few days. A classic clinical pearl is that a headache is often one of the first symptoms to appear, typically within 12 to 24 hours after the last intake of caffeine.

## Correct Answer: D.