This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Growth, Subject name: Pediatrics
Which X ray is used to determine bone age in a 3 year old child? 
Select and explain. 
A. Knee, B. Wrist, C. Shoulder, D. Ankle
B: Wrist
Answer: B: Wrist 
Explanation: To determine the skeletal age in infants between 3 and 9 months age, X ray of shoulder is most useful. X ray of hand and wrist is most useful for children between 1 and 13 year age.