A 52-year-old man complains of impotence. On physical examination, he has an elevated jugular venous pressure, S3 gallop, and hepatomegaly. He also appears tanned, with pigmentation along skin folds. He has joint pain and bony overgrowth primarily affecting the second and third metacarpophalangeal joints bilaterally. The plasma glucose is 250 mg/dL, and liver enzymes are elevated. Which of the following studies will help establish the diagnosis?