NBME Step 2 CK Form 8 X-ray of the sacroiliac joints (NBME Answers)

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Young man who presents with inflammatory back pain (worse in the morning but improves over course of the day) who has limited range of motion of the back (consistent with “bamboo spine” physiology), all of which is most consistent with ankylosing spondylitis that can be diagnosed with x-ray or MRI of the sacroiliac joints
Key idea: Indications for x-ray in setting of low back pain is (1) Osteoporosis or compression fracture (2) Suspected malignancy (3) Ankylosing spondylitis
Key idea: Indications for MRI in setting of low back pain is (1) Sensory/motor deficits (2) Cauda equina syndrome (3) Suspected epidural abscess or infection

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seagull THe question said initial step. I thought this was a clinical dx that required elevated ESR, CRP. In reality we would order these and have him get an x-ray. I'm not sure if we can reliably dx Ankylosing Spondylitis unless we have the ESR unless the x-ray clearly shows that bones are fusing. THis is a younger guy too. +1

kingfriday There was a uworld question that mirrors this if you use the search function you can probably find it. The reasoning they had there said that acute phase markers are usually elevated in AS but they have low specificity for establishing the dx. BONE SCAN - not good for AS, but it is good for osteomyelitis, suspected fractures, and neoplasms > MRI indicated for neurological s/sx +3

spiroskeet Just found that UWorld question – it asked which would be most likely to establish a diagnosis in the patient. In that case, X-ray of SI joints is the right answer. However, the NBME question asked for initial step. My first step would probably be to order an ESR. It's nonspecific, but ESR is pretty much always nonspecific, so why would you ever order it? +2

charcot_bouchard Here ESR doesnt make diagnosis, doesnt change managment (like treatment for ESR high another for low). SO no need of ESR. U do nonspecific test when it changes mx or saves/delays a bothersome test like temporal artery biopsy. +1