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To the Editor: Henderson et al. (March 21 issue)1 provide a detailed analysis of a 58-year-old woman with severe vitamin D deficiency related to vitamin D–binding protein deficiency. According to the case report, the patient had concomitant ankylosing spondylitis, evidenced by the presence of bilateral sacroiliitis, bridging syndesmophytes, and enthesopathy. We question whether this patient’s debilitating spinal arthritis was a musculoskeletal manifestation of unrecognized metabolic bone disease rather than a coincidental autoimmune disease, as reported by the authors. The axial manifestations of certain forms of osteomalacia can mimic ankylosing spondylitis, owing to the presence of calcifying enthesopathies,2 irregular sacroiliac joint . . .