**Core Concept:**
Dystonia is a movement disorder characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive, movements or postures. It can be caused by various factors such as genetic, neurological, or environmental influences. In this scenario, the child's poor school grades could indicate a combination of neurological and developmental factors. Slit examination is a tool used to assess the optic nerve head and can provide valuable information in neuro-ophthalmological disorders.

**Why the Correct Answer is Right:**
The correct answer is C: Full-field electroretinogram (ERG) because it is the most appropriate initial investigation for evaluating the patient's symptoms. A full-field ERG helps to assess the overall function of the retinal ganglion cells and bipolar cells, which are essential for vision and are involved in dystonia. It is especially useful in detecting early-stage retinal dysfunction and optic nerve involvement.

**Why Each Wrong Option is Incorrect:**
A) Vitamin B12 deficiency (methylmalonic aciduria) is one of the common causes of dystonia, but a full-field ERG is not related to this investigation.
B) A visual evoked potential (VEP) is an electrophysiological test used to evaluate the visual pathway from the optic nerve to the brain, but it is not specific for dystonia and does not directly assess retinal ganglion cells and bipolar cells.
D) A visual acuity test measures the child's visual acuity, which is an important aspect of vision but it does not address the retinal ganglion cells and bipolar cells dysfunction, which is crucial for dystonia evaluation.

**Clinical Pearl:**
Investigating dystonia with visual acuity test and VEP may provide some information, but the full-field ERG is the most comprehensive test for evaluating retinal ganglion cells and bipolar cells function, which are essential for vision and contribute to dystonia symptoms.

**Correct Answer Explanation:**
A full-field ERG is the correct answer because it directly assesses retinal ganglion cells and bipolar cells function. These cells are involved in dystonia symptoms, and their dysfunction can lead to visual disturbances such as those described in the question. The other options, like vitamin B12 deficiency and VEP, focus on different aspects of vision but do not address the specific cells involved in dystonia symptoms. Visual acuity test measures visual acuity, which is important but does not directly evaluate the retinal ganglion cells and bipolar cells function.