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
Most common cause of hyperparathyroidism is- 
Select and explain. 
A. Solitary adenoma, B. Chief cell hyperplasia, C. Multiple adenoma, D. Werner's syndrome
A: Solitary adenoma
Answer: A: Solitary adenoma 
Explanation: a single abnormal gland is the cause in 80% of patients , abnormality is usually beningn ( Harrison 17 pg 2380)