This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
[{'content': 'Most common cause of Hyperparathyroidism is? \nSelect and explain. \nA. Parathyroid adenoma, B. Parathyroid hyperplasia, C. Thyroid carcinoma, D. Medullary carcinoma thyroid', 'role': 'user'}, {'content': "Answer: A \nExplanation: Primary hyperparathyroidism is caused by autonomous secretion of PTH, usually by a single parathyroid adenoma, which can vary in diameter from a few millimetres to several centimetres. It should be distinguished from secondary hyperparathyroidism, in which there is a physiological increase in PTH secretion to compensate for prolonged hypocalcaemia (such as in vitamin D deficiency, p. 1126), and from teiary hyperparathyroidism, in which continuous stimulation of the parathyroids over a prolonged period of time results in adenoma formation and autonomous PTH secretion (Box 20.39). This is most commonly seen in individuals with advanced chronic kidney disease . DAVIDSON'S PRINCIPLES AND PRACTICE OF MEDICINE 22ND EDITION PAGE NO-769", 'role': 'assistant'}]