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: Medicine
[{'content': 'The commonest cause of congenital hypothyroidism- \nSelect and explain. \nA. Thyroid dysgenesis, B. Pendred syndrome, C. Defective release, D. Deficiency of deiodinase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]