Ulnar claw

A 23-year-old woman comes to the office because of reduced movement of her right hand. Her hand takes a claw-like appearance even when relaxed. She has noticed a hollowed-out area between the thumb and forefinger. Physical examination shows hyper-extension of her metacarpo-phalangeal joints with flexion of the distal and proximal interphalangeal joints of the 4th and 5th fingers. Which of the following is the most likely diagnosis?