Okay, let's tackle this question. The patient is a 30-year-old man with paranoid thoughts. He suspects his wife is having an affair with his boss, thinks his friend is involved from abroad with technology support, and believes people talk ill of him. Friends have tried to convince him, but he's not convinced. He's otherwise normal without thought disorder or inappropriate behavior.

First, I need to recall the diagnostic criteria for different psychiatric conditions. The key here is persistent paranoid delusions without other symptoms like hallucinations or disorganized speech. Delusional disorder is characterized by one or more delusions that persist for at least a month. The delusions are non-bizarre and could occur in real life, like infidelity or being spied on. The patient functions normally otherwise, which fits delusional disorder.

Other options to consider: Schizophrenia requires two or more symptoms (delusions, hallucinations, disorganized speech, etc.) for at least a month. Since he doesn't have other symptoms, this is less likely. Paranoid personality disorder involves pervasive distrust and suspiciousness, but the delusions here are more specific and persistent, which points more to delusional disorder. Major depressive disorder with psychotic features would include depression plus delusions, but the question states he's otherwise normal, so no depression is mentioned.

The clinical pearl here is that delusional disorder is diagnosed when there are non-bizarre delusions without the other symptoms required for schizophrenia. The key differentiator is the absence of other psychotic symptoms and the presence of a single, persistent delusion. Also, the patient's functioning is not significantly impaired beyond the delusional content.

**Core Concept**
This question tests the differentiation between **delusional disorder** and other psychotic disorders. Delusional disorder is characterized by **non-bizarre delusions** (beliefs that could occur in real life) lasting ≥1 month, with no prominent hallucinations or disorganized behavior. The key distinction from schizophrenia lies in the absence of additional psychotic features.

**Why the Correct Answer is Right**
The patient exhibits **persistent paranoid delusions** (infidelity, conspiracies) without hallucinations, disorganized speech, or catatonia. Delusional disorder (persecutory type) fits because the delusions are **non-bizarre**, the individual functions normally otherwise, and there is **no evidence of schizophrenia** (which requires ≥2 core symptoms). The lack of thought disorder or inappropriate behavior further supports this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** *Schizophrenia* requires ≥2 symptoms (delusions, hallucinations, disorganized speech, etc.) for ≥6 months. The patient lacks additional symptoms like hallucinations or disorganized behavior.
**Option B:** *Paranoid personality disorder* involves pervasive distrust and suspiciousness but lacks fixed, unshakable delusions. The patient’s beliefs are **fixed and non-fluctuating**, not just suspiciousness.
**Option C:** *Brief psychotic disorder* requires acute onset and resolution within 1 month. The 2-month duration exceeds this threshold.

**Clinical Pearl / High-Yield Fact**
Delusional disorder is often underdiagnosed and mistaken for schizophrenia. The **absence of other psychotic features** and **normal functioning** outside the delusional content are critical for differentiation. Remember: *“One delusion, no

✓ Correct Answer: B. Persistent delusion disorder