A 30-year-old man has had cramping abdominal pain and bloody diarrhea for the past 4 days. On physical examination, there is diffuse tenderness on palpation of the abdomen. Bowel sounds are present. There are no masses and no organomegaly. A stool culture is positive for Shigella flexneri. The episode resolves spontaneously within 1 week after onset. Six weeks later, he has increasingly severe lower back pain. Physical examination now shows stiffness of the lumbar spine and tenderness of the sacroiliac joints. He is treated with nonsteroidal anti-inflammatory agents. Two months later, the back pain recurs, and he complains of redness of the right eye and blurred vision. Serologic testing for which of the following is most likely to be positive in this patient?
A 30-year-old man has had cramping abdominal pain and bloody diarrhea for the past 4 days. On physical examination, there is diffuse tenderness on palpation of the abdomen. Bowel sounds are present. There are no masses and no organomegaly. A stool culture is positive for Shigella flexneri. The episode resolves spontaneously within 1 week after onset. Six weeks later, he has increasingly severe lower back pain. Physical examination now shows stiffness of the lumbar spine and tenderness of the sacroiliac joints. He is treated with nonsteroidal anti-inflammatory agents. Two months later, the back pain recurs, and he complains of redness of the right eye and blurred vision. Serologic testing for which of the following is most likely to be positive in this patient?
💡 Explanation
**Core Concept**
The patient's presentation of recurrent lower back pain, sacroiliac joint tenderness, and eye inflammation suggests an association with an autoimmune inflammatory disorder. This condition is likely linked to a genetic predisposition, specifically a specific human leukocyte antigen (HLA) type.
**Why the Correct Answer is Right**
The patient's symptoms are characteristic of ankylosing spondylitis (AS), a chronic inflammatory disease primarily affecting the axial skeleton. AS is strongly associated with the HLA-B27 allele, which is present in approximately 90% of patients with this condition. The presence of HLA-B27 increases the risk of developing AS and other spondyloarthropathies, such as reactive arthritis (previously known as Reiter's syndrome). In reactive arthritis, the gastrointestinal and genitourinary symptoms often precede the joint manifestations, which is consistent with the patient's initial presentation of bloody diarrhea caused by Shigella flexneri infection. The subsequent development of eye inflammation, known as uveitis, further supports the diagnosis of AS.
**Why Each Wrong Option is Incorrect**
**Option A:** Borrelia burgdorferi is the causative agent of Lyme disease, which can cause arthritis, but it is not associated with the patient's symptoms or the HLA-B27 allele.
**Option B:** Chlamydia trachomatis is a common cause of reactive arthritis, but the patient's serologic testing would not be positive for this organism, as the infection has resolved.
**Option C:** Epstein-Barr virus is associated with various autoimmune conditions, including multiple sclerosis and lupus, but it is not linked to the patient's symptoms or the HLA-B27 allele.
**Clinical Pearl / High-Yield Fact**
The HLA-B27 allele is a genetic marker that can be used to predict the risk of developing ankylosing spondylitis and other spondyloarthropathies. However, not all individuals with HLA-B27 will develop these conditions, and not all patients with ankylosing spondylitis will test positive for HLA-B27.
**Correct Answer Line**
✓ Correct Answer: D. HLA-B27
✓ Correct Answer: D. HLA-B27
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