A 34-year-old man comes to the outpatient clinic reporting a 6-month history of intermitte… — USMLE Step 3

A $34$ -year-old man comes to the outpatient clinic reporting a $6$ -month history of intermittent abdominal cramps, non-bloody diarrhea, and a $5$ -kg weight loss. An outside colonoscopy performed two months ago reportedly revealed isolated ileocecal ulcerations. He also reports having recurrent, painful sores in his mouth and on his scrotum over the past year, which heal spontaneously but frequently return. Currently, he is experiencing severe pain and swelling in his right knee that began three days ago. He has been taking over-the-counter ibuprofen daily for the joint pain. Vital signs are within normal limits. Physical examination shows a warm, swollen right knee with a restricted range of motion. Examination of the oral mucosa reveals three shallow, painful ulcers with erythematous borders on the buccal mucosa. The patient's left forearm exhibits a $1.5$ -cm pustule surrounded by an erythematous halo; he states this lesion developed rapidly after a routine blood draw at that site two days ago. Laboratory studies show:

Hemoglobin: $11.2$ g/dL
Leukocyte count: $11,500/\mu L$
Platelet count: $450,000/\mu L$
Antinuclear antibody (ANA): $1:40$ (normal: $<1:40$ )
C-reactive protein: $45$ mg/L (normal: $<5$ mg/L)

Based on the constellation of clinical findings, which of the following is the most likely diagnosis?

A
Crohn disease
B
Reactive arthritis
Behçet syndromeAnswer
D
Systemic lupus erythematosus
E
NSAID-induced enteropathy

Answer

Behçet syndrome

The correct answer is Behçet syndrome. Behçet syndrome is a multi-systemic, variable-vessel vasculitis characterized classically by recurrent oral aphthous ulcers, painful genital ulcers, and uveitis. Gastrointestinal involvement occurs in a significant subset of patients and frequently presents with mucosal ulcerations heavily concentrated in the ileocecal region, closely mimicking Crohn disease both clinically (abdominal pain, diarrhea) and endoscopically. The key to formulating the correct differential diagnosis in this patient lies in recognizing the mucocutaneous findings: painful genital ulcers are not a typical feature of Crohn disease, and the rapid development of a pustule at the site of minor skin trauma (the recent blood draw) represents the pathergy phenomenon, which is highly specific for Behçet syndrome. The patient's monoarthritis and elevated inflammatory markers complete the clinical picture.

Step-by-Step Solution

Analyze the gastrointestinal and systemic symptoms.

The patient has chronic diarrhea, weight loss, and ileocecal ulcerations, paired with systemic signs like oral ulcers and monoarthritis.

Establishing the base syndrome is necessary to anchor the differential diagnosis, which initially points heavily toward inflammatory bowel disease.

Identify findings that are atypical for the most common preliminary diagnosis (Crohn disease).

The patient also has recurrent painful genital ulcers and a pustule at a venipuncture site (pathergy).

Relying purely on the GI and oral symptoms leads to premature closure; identifying discriminatory symptoms broadens the differential accurately.

Synthesize all clinical clues to identify the single unifying diagnosis.

The combination of oral and genital aphthous ulcers, pathergy phenomenon, arthritis, and Crohn-like ileocecal ulcerations defines Behçet syndrome.

Behçet syndrome is a multi-system vasculitis whose gastrointestinal manifestations are a known, exact mimic of Crohn disease, distinguishable primarily by specific mucocutaneous findings.

Evaluate laboratory results to rule out distractors.

The ANA titer is 1:40, which is borderline/normal and non-diagnostic; inflammatory markers (CRP, WBC) are elevated, consistent with a systemic flare.

Ruling out SLE confirms that the mild ANA abnormality should not shift the diagnostic focus away from the clear clinical syndrome.

Key Concept

Differentiating Behçet syndrome from Crohn disease using specific mucocutaneous findings (genital ulcers, pathergy).

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