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
Most common cause of hyperparathyroidism - 
Select and explain. 
A. Iatrogenic, B. Medullary carcinoma thyroid, C. Parathyroid adenoma, D. Parathyroid hyperplasia
C: Parathyroid adenoma
Answer: C: Parathyroid adenoma 
Explanation: None