Final answer:
The correct code for a follow-up surgery for a malunion of a Salter-Harris type 2 fracture of the humerus is S49.021D, indicating a subsequent encounter for fracture healing.
Step-by-step explanation:
The code for a follow-up surgery of a Salter-Harris type 2 fracture of the upper end of the humerus of the right arm due to a malunion is S49.021D. This code indicates a subsequent encounter for a closed fracture with routine healing. The use of the seventh character 'D' is specific for encounters after the patient has completed active treatment for the fracture and is receiving routine care for the fracture during the healing or recovery phase. The Salter-Harris fracture classification system is used to describe fractures that involve the epiphyseal plate or growth plate in children, with type 2 being one of the most common types, which includes fractures that travel through the growth plate and break off a piece of the metaphysis.