**Core Concept**
Neurocardiogenic syncope, also known as vasovagal syncope, is a common cause of fainting characterized by a sudden drop in heart rate and blood pressure, often triggered by emotional stress or prolonged standing. This condition is related to the autonomic nervous system's dysfunction, particularly involving the vagus nerve and its effect on the heart.

**Why the Correct Answer is Right**
The least useful investigation for neurocardiogenic syncope is an electroencephalogram (EEG). This is because EEGs are used to diagnose and monitor seizure disorders and do not provide relevant information about the autonomic nervous system's function or the heart's electrical activity in the context of neurocardiogenic syncope. In contrast, other investigations like tilt table testing, heart rate variability analysis, and serum electrolyte levels may be more useful in diagnosing and managing this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** **Holter monitor** - A Holter monitor is a portable device used to record the heart's electrical activity over a 24-hour period, which can be useful in diagnosing arrhythmias and other cardiac conditions. However, it is not the least useful investigation for neurocardiogenic syncope.
**Option B:** **Tilt table testing** - Tilt table testing is a useful investigation for neurocardiogenic syncope, as it can help diagnose the condition by reproducing the symptoms and monitoring the heart's response to changes in posture.
**Option C:** **Serum electrolyte levels** - Serum electrolyte levels, particularly potassium and sodium, can be useful in diagnosing and managing neurocardiogenic syncope, as electrolyte imbalances can trigger or exacerbate the condition.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that neurocardiogenic syncope is often associated with an underlying anxiety disorder, and treating the anxiety can help prevent future episodes.

**Correct Answer: A. EEG**