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Difficulty: HardSchizophrenia and Psychotic Disorders

A 45-year-old man presents to a dermatologist for the fifth time in a year. He is convinced that he has a severe parasitic infection of his skin, specifically claiming that microscopic worms are burrowing under his epidermis and multiplying. During the visit, he triumphantly shows the physician a small matchbox containing lint, scabs, and skin flakes, asserting they are "specimens" of the parasites. He reports that this infestation began 7 months ago. He strongly denies any depressed mood, hearing voices, or other unusual experiences. He continues to work successfully as a mechanical engineer and maintains a healthy relationship with his wife, who states he is completely normal except for his "obsession with his skin." Physical examination reveals numerous linear excoriations on his forearms and lower legs from chronic scratching. A comprehensive toxicology screen is negative. Which of the following is the most likely diagnosis?

  1. Delusional disorderAnswer
  2. B
    Schizophreniform disorder
  3. C
    Brief psychotic disorder
  4. D
    Obsessive-compulsive disorder
  5. E
    Illness anxiety disorder

Answer

The most likely diagnosis is delusional disorder.
The correct answer is delusional disorder. This patient presents with a classic case of delusional parasitosis (Ekbom syndrome), a specific manifestation of delusional disorder, somatic type. Delusional disorder is characterized by the presence of one or more delusions for at least 1 month. A key distinguishing feature is that, apart from the direct impact of the delusion (e.g., scratching the skin, visiting dermatologists), the patient's overall psychosocial functioning is relatively preserved. Furthermore, the patient does not meet the criteria for schizophrenia (he lacks prominent hallucinations, disorganized speech, disorganized behavior, and negative symptoms). The "matchbox sign"—bringing in lint or skin debris as proof of the infestation—is a pathognomonic clinical clue for this condition.

Step-by-Step Solution

1
Identify the primary psychiatric symptom.
The patient has a fixed, false belief that he is infested with parasites, evidenced by bringing a matchbox of "specimens" (Ekbom syndrome/delusional parasitosis).
Establishing the core symptom as a somatic delusion is the first step in formulating the differential diagnosis.
2
Evaluate the duration of symptoms and the patient's functional status.
The delusion has been present for 7 months. Despite the severity of the belief, his occupational and social functioning remain entirely intact.
Duration is critical for distinguishing between psychotic disorders (e.g., brief psychotic disorder vs. schizophreniform vs. schizophrenia/delusional disorder).
3
Differentiate from related psychiatric conditions based on specific criteria.
The absence of prominent hallucinations, disorganized speech, or negative symptoms rules out schizophrenia. Intact insight rules out OCD and illness anxiety disorder. The 7-month duration rules out brief psychotic disorder and schizophreniform disorder.
Systematic elimination based on DSM-5 criteria confirms the diagnosis of delusional disorder, somatic type.

Key Concept

Patients with delusional disorder exhibit one or more delusions for at least 1 month without meeting the criteria for schizophrenia, and their psychosocial functioning is not markedly impaired outside of the delusion.
Estimated Time:1m 30s
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Topics

Schizophrenia and Psychotic Disorders

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