Review Question - QID 7895

A 66-year-old woman with known poorly controlled rheumatoid arthritis reports that for the past 4 weeks she has been unable to extend the metacarpophalangeal (MCP) joints of her right hand index, middle, ring and little fingers. She cannot hyperextend the thumb interphalangeal joint. Active wrist extension is possible, but shows radial deviation. Examination reveals mild synovitis at the wrist and MCP joints of the affected hand. There is no ulnar deviation at the MCP joints with normal alignment. When the MCP joints are passively extended, the patient is unable to maintain them in this position. There is no piano key sign at the distal ulna. Passive wrist motion shows a normal tenodesis effect. Which of the following would most likely confirm your diagnosis?