A 6-year-old boy presents to the emergency room after a fall from his bicycle with an isolated left elbow injury. A radiographic series is obtained and the fracture line is only appreciated on the internal oblique view. The greatest distance between the humerus and the fracture fragment is measured to be 2 mm. The patient's mother is strongly opposed to operative intervention and he is placed in a cast. However, the resident explains to her that this injury has the highest rate of nonunion about the elbow and that close follow-up will be necessary. Which of the following injuries is described in this scenario?