A 7-year-old girl is brought to the outpatient clinic 10 days after a supracondylar fracture of the humerus that was treated with closed reduction and posterior slab immobilization. The fracture was undisplaced on initial X-ray. On re-examination today, she has developed a 'claw hand' deformity with inability to flex the interphalangeal joints of the fingers. Sensory examination shows loss of sensation over the medial 1.5 fingers. What is the most likely nerve injury?