A 32-Year-Old Woman Comes to Her Physician Because of Increasing

A 32-year-old woman comes to her physician because of increasing back pain for the past 10 months. The pain is worse in the morning when she wakes up and improves with activity. She used to practice yoga, but stopped 5 months ago as bending forward became increasingly difficult. She has also had bilateral hip pain for the past 4 months. She has not had any change in urination. Examination shows the range of spinal flexion is limited. Flexion, abduction, and external rotation of bilateral hips produces pain. Which of the following is most likely to confirm the diagnosis?

A) Complete blood count
B) Erythrocyte sedimentation rate
C) Nerve conduction studies
D) Rheumatoid factor level
E) X-ray of the pelvis