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The most useful test of those provided in the choices is (C) – the HLA-B27 antigen test.
(For a normal image of a the lumbar spine, click here)
This patient has enteropathic spondylitis, or ankylosing spondylitis associated with inflammatory bowel disease (which will be confirmed by colonoscopy/biopsy). These diseases are part of the seronegative spondyloarthropathies, for which we use the mnemonic PAIR (Psoriatic arthritis, ankylosing spondylitis, Inflammatory bowel disease [Crohn's + ulcerative colitis], Reiter’s syndrome/Reactive arthritis). These multi-systemic inflammatory conditions are referred to as “seronegative” because they are not associated with rheumatoid factor or other autoantibodies.
They are, however, associated with HLA-B27, an antigen found on the surface of human leukocytes (white blood cells). This trait is genetically determined, and so these conditions tend to display heritability (a father with Crohn’s disease would not be a surprising discovery). Only a small percentage (often quoted at 7%) of the population carries HLA-B27, and an even smaller number (<1%) have ankylosing spondylitis – so carrying the gene does not mean one will develop the disease. However, >90% of those with AS will also have HLA-B27 – thus, in someone who presents with symptoms consistent with the disease, testing for this antigen will aid in its proper diagnosis.
The patient’s musculoskeletal problems can be attributed to the AS, while his weight loss and occult blood in the stool can be attributed to IBD. Chronic blood loss in the stool can lead to anemia, but the elevated platelets are the result of an inflammatory process (additionally, you may expect the patient’s C-reactive protein to be elevated).
Treatment for a patient like this will revolve around regular follow up exams and x-rays, as well as physical therapy and TNF-blockers (e.g. adalimumab/Humira).
Blood culture (A) and stool culture (B) are not the best answer in this case because of the strong evidence in favor of IBD & AS. It is important to always keep infectious processes in mind, but the length of time this patient has been noticing symptoms is inconsistent with most infectious etiologies.
A B12 absorption study (D) would be more useful in determining the etiology of megaloblastic/pernicious anemia. The characteristic patient is an elderly female with fatigue, pseudodementia or even subacute combined degeneration of the spinal cord (sensory & motor findings).
The next best step for this patient is not gluten avoidance (E). It is not likely that he has Celiac disease, although fatigue and anemia are consistent with its presentation. Signs & symptoms of malabsorption are more typical in those with Celiac disease.
Antinuclear antibodies (F) are seen in patients with lupus. The demographics in this case shouldn’t lead you in this direction – lupus is seen primarily in females, and AS is seen more typically in males.