**Core Concept**
The patient's symptoms suggest a transient ischemic attack (TIA), which is a temporary period of neurological dysfunction caused by a transient reduction in blood flow to the brain, often due to atherosclerotic plaque rupture or embolism. A thorough evaluation is necessary to identify the underlying cause and prevent future strokes.

**Why the Correct Answer is Right**
The patient's history of transient episodes of sudden loss of vision in one eye suggests a possible source of emboli, such as a carotid artery stenosis or atrial fibrillation. A non-invasive imaging test like Duplex ultrasound or carotid Doppler ultrasound can assess the degree of carotid artery stenosis and detect potential sources of emboli. This test is non-invasive, quick, and provides valuable information about the patient's risk of future strokes.

**Why Each Wrong Option is Incorrect**
**Option A:** A head CT scan may be useful in ruling out acute hemorrhage or large vessel occlusion, but it is not the next best step in this case, as the patient's neurological examination is normal and the symptoms are suggestive of a TIA.
**Option B:** An MRI with diffusion-weighted imaging may be useful in detecting small vessel disease or cerebral vasculitis, but it is not the most appropriate next test in this case, as the patient's symptoms are more suggestive of a large vessel disease.
**Option C:** An electrocardiogram (ECG) may be useful in detecting atrial fibrillation or other cardiac arrhythmias that may be contributing to the patient's symptoms, but it is not the most appropriate next test in this case, as the patient's symptoms are more suggestive of a large vessel disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that TIA symptoms typically resolve within 24 hours, and a thorough evaluation is necessary to identify the underlying cause and prevent future strokes.

**Correct Answer: C. ECG.**