Final answer:
Child abuse, specifically non-accidental trauma, is the most common cause of a corner metaphyseal fracture in a child younger than 12 months of age.
Step-by-step explanation:
The most common cause of a corner metaphyseal fracture in a child younger than 12 months of age is child abuse, specifically non-accidental trauma (NAT). In cases of NAT, the fracture occurs when there is a twisting or pulling force applied to the limb, causing the metaphysis (the wider part of the bone near the growth plate) to break.