**Core Concept**
Schizophrenia is a chronic mental disorder characterized by disruptions in thought processes, perceptions, and emotional responsiveness. Neuroleptics, also known as antipsychotics, are a class of medications commonly used to treat psychotic symptoms of schizophrenia, such as hallucinations and delusions.

**Why the Correct Answer is Right**
The patient's presentation of sadness, reduced speech, and social withdrawal could be indicative of several possible causes. However, the statement "He is reacting to external stimuli" is unlikely to be the correct explanation. Antipsychotic-induced extrapyramidal symptoms (such as parkinsonism) or persistent negative symptoms of schizophrenia could be contributing factors. Major depression is also a plausible cause, especially considering the patient's social withdrawal and sadness.

**Why Each Wrong Option is Incorrect**
**Option A:** Parkinsonism is a common side effect of long-term antipsychotic use, characterized by motor symptoms such as tremors, rigidity, and bradykinesia. It is a plausible explanation for the patient's presentation.
**Option B:** Major depression is a common comorbidity with schizophrenia, and the patient's symptoms of sadness and social withdrawal could be indicative of a depressive episode.
**Option C:** Negative symptoms of schizophrenia, such as social withdrawal and apathy, can persist even after the initiation of antipsychotic treatment. This is a common challenge in schizophrenia management.

**Clinical Pearl / High-Yield Fact**
Be aware of the distinction between positive symptoms of schizophrenia (e.g., hallucinations, delusions) and negative symptoms (e.g., social withdrawal, apathy). Negative symptoms can be challenging to treat and may require additional therapeutic interventions.

**Correct Answer:**
✓ Correct Answer: D. He is reacting to external stimuli