A 38-year-old, previously healthy, nonsmoking man has been referred to you for evaluation of digital clubbing (Fig 1, 2
), which was noted during a routine pre-employment physical examination. He also has an abnormal knee radiograph (Fig 3
). His only complaint is aching pain and the feeling of swelling just above his knees for about 6 months. He also volunteers the information that his fingers look no differently now than they did when he was a teenager. Physical examination is normal, except that he appears older than his stated age and has skin changes and clubbing of 10 digits, as shown in the accompanying figures. Which of the following tests will be needed to establish his diagnosis?