**Core Concept**
The vacant stare in a school-going boy without fever, seizures, or neurological deterioration suggests a psychiatric or neurodevelopmental disorder. The vacant stare is a characteristic feature of catatonia, a condition marked by immobility, rigidity, and a lack of responsiveness to the environment.

**Why the Correct Answer is Right**
Catatonia is a neuropsychiatric disorder characterized by a range of symptoms, including immobility, mutism, rigidity, and a vacant stare. The vacant stare in catatonia is due to a disruption in the normal flow of neural activity in the brain, particularly in the prefrontal cortex and basal ganglia. This disruption can be caused by various factors, including genetic predisposition, stress, and certain medical conditions. In this case, the absence of fever, seizures, or neurological deterioration suggests that the vacant stare is likely due to a psychiatric or neurodevelopmental disorder rather than a medical condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the vacant stare as a primary symptom. While autism spectrum disorder (ASD) can involve social withdrawal and communication difficulties, it is not typically characterized by a vacant stare.
**Option B:** This option is incorrect because attention deficit hyperactivity disorder (ADHD) is primarily characterized by symptoms of inattention, hyperactivity, and impulsivity, which do not typically include a vacant stare.
**Option C:** This option is incorrect because schizophrenia can involve a range of symptoms, including hallucinations, delusions, and disorganized thinking, but a vacant stare is not a primary feature of the disorder.
**Option D:** This option is incorrect because obsessive-compulsive disorder (OCD) is primarily characterized by symptoms of obsessions and compulsions, which do not typically involve a vacant stare.

**Clinical Pearl / High-Yield Fact**
The vacant stare in catatonia is often accompanied by other characteristic features, including immobility, rigidity, and mutism. It is essential to differentiate catatonia from other conditions that may present with similar symptoms, such as schizophrenia or autism spectrum disorder.

**Correct Answer:** C. Catatonia.