**Core Concept**
Schizophrenia is a chronic and severe mental disorder characterized by distortions in thinking, perception, emotions, language, sense of self, and behavior. Common symptoms include hallucinations, delusions, disorganized thinking, and negative symptoms such as apathy or lack of emotion.

**Why the Correct Answer is Right**
The patient's symptoms of delusions (believing his neighbors are plotting against him, and his wife has been replaced by a double) and hallucinations (hearing his neighbors sneeze) are classic examples of positive symptoms of schizophrenia. The presence of these symptoms, along with a 10-year history, suggests a diagnosis of schizophrenia. The patient's relatively good grooming and occasional alcohol consumption do not rule out schizophrenia.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect as it does not match the patient's symptoms. Schizoaffective disorder involves a combination of schizophrenia and mood disorder symptoms, which are not present in this case.
**Option B:** This is incorrect as it does not fit the patient's symptoms or history. Schizophreniform disorder is a short-term (less than 6 months) psychotic disorder, which does not align with the patient's 10-year history.
**Option C:** This is incorrect as it does not match the patient's symptoms. Brief psychotic disorder is a short-term (less than 1 month) psychotic disorder, which does not align with the patient's 10-year history.
**Option D:** This is incorrect as it does not match the patient's symptoms. Substance-induced psychotic disorder would require evidence of substance use or withdrawal, which is not provided in the scenario.

**Clinical Pearl / High-Yield Fact**
The positive symptoms of schizophrenia (hallucinations and delusions) are often more easily identified than the negative symptoms (apathy or lack of emotion). A key aspect of diagnosing schizophrenia is ruling out other potential causes of these symptoms, such as substance use or other psychiatric disorders.

**Correct Answer:** **C.**