A couple comes into the emergency room where the husband is convinced of his wife's infidelity. What is the likely diagnosis for this scenario?

In this scenario, the husband's strong conviction about his wife's infidelity aligns well with the characteristics of delusional disorder. This condition is marked by the presence of one or more persistent delusions that last for at least a month. Delusions involve firmly held beliefs that are contradicted by reality but are resistant to reason or evidence.

The specific delusion of infidelity, known as the "Othello syndrome" or "the belief in being betrayed by a partner," fits perfectly within the realm of delusional disorder, particularly the subtype known as the "jealous type." Individuals with this disorder may interpret benign behaviors of their partners as evidence of infidelity, leading to significant distress and functional impairment.

In contrast, schizophrenia involves a broader range of symptoms, including hallucinations, disorganized thinking, and affected functioning, which are not indicated by the husband's focused belief. Major depressive disorder with psychotic features would typically include symptoms of depression alongside psychotic symptoms, not exclusively a single delusion. Lastly, dementia tends to present with a decline in cognitive abilities and various changes in behavior but does not specifically characterize the scenario of a single persistent false belief regarding infidelity. Thus, the context provided strongly supports delusional disorder as the appropriate diagnosis.