Okay, let's tackle this question. The patient is a 30-year-old man who's suspicious that his wife is having an affair with his boss. He also thinks his friend is involved from abroad, even after being convinced otherwise. He doesn't have hallucinations or other socially inappropriate behaviors.

First, I need to recall the different psychiatric disorders. The key here is the delusional content. Delusional disorder comes to mind because he's holding a fixed, false belief without other symptoms like hallucinations. The other options might be schizophrenia, but he doesn't have the other symptoms like disorganized speech or negative symptoms. Paranoid schizophrenia does have paranoid delusions, but again, there are other features. Adjustment disorder might not fit because it's more about stress response without delusions. Schizoaffective disorder would have mood symptoms too.

So the core concept is delusional disorder. The correct answer is Delusional Disorder. The other options are ruled out because they either have additional symptoms or don't fit the presentation. The clinical pearl here is to remember that delusional disorder is characterized by persistent delusions without other psychotic features.

**Core Concept**
This question tests the differential diagnosis of persistent delusional thinking without accompanying hallucinations or disorganized behavior. The key differentiator is the presence of **delusional disorder (paranoid type)**, characterized by nonbizarre delusions (e.g., infidelity, conspiracies) that are fixed and not secondary to another psychiatric condition.

**Why the Correct Answer is Right**
The patient exhibits **delusional disorder (paranoid type)** due to persistent, fixed delusions (wife’s infidelity, friend’s involvement) without hallucinations, disorganized speech/thinking, or catatonic behavior. Delusions are nonbizarre (plausible in real life) and centered on a theme of persecution or suspicion. The absence of other psychotic features (e.g., flat affect, avolition) rules out schizophrenia or schizoaffective disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** *Schizophrenia* requires two or more symptoms (e.g., delusions, hallucinations, disorganized speech) for ≥6 months, which is not met here.
**Option B:** *Paranoid personality disorder* involves pervasive distrust but lacks the fixed delusions described.
**Option C:** *Schizoaffective disorder* requires a major mood episode concurrent with psychotic symptoms, which is absent here.
**Option D:** *Delusional disorder (jealous type)* is a subtype but is now subsumed under "paranoid type" in DSM-5.

**Clinical Pearl / High-Yield Fact**
Delusional disorder is often misdiagnosed as schizophrenia due to overlapping delusions. **Key exam trap**: Remember that delusional disorder lacks the *negative symptoms* (e.g., anhedonia) and *disorganized behavior* seen in schizophrenia. Use the mnemonic **"1 delusion, no hallucinations = delusional disorder"**.

**Correct Answer: C. Delusional Disorder (Paranoid Type)**

✓ Correct Answer: C. Persistent delusional disorder