This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Isotope used for RAIU is ? 
Select and explain. 
A. 1-123, B. 1-137, C. 1-125, D. None
A: 1-123
Answer: A: 1-123 
Explanation: Ans. is'a'i.e., I-123Radio-active iodine uptake (RAIU) test is used to measure thyroid function.Usual isotope of radioiodine used b I-123.