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: Pathology
The commonest cause of raised serum calcium is - 
Select and explain. 
A. Ectopic secretion, B. Parathyroid hyperplasia, C. Parathyroid adenoma, D. Parathyroid carcinoma
C: Parathyroid adenoma
Answer: C: Parathyroid adenoma 
Explanation: Primary hyperparathyroidism is the most common cause of hypercalcemia overall.
The frequency of the various parathyroid lesions underlying the hyperfunction is as follows : -

Parathyroid adenoma - 75% to 80%
Primary hyperplasia - 10% to 15%
​Parathyroid carcinoma- < 5%

After hyperparathyroidism, malignancy is the 2nd most common cause of hypercalcemia by one of the following mechanisms.

Osteolysis
Secretion of parathormone related peptide.