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What is The commonest cause of raised serum calcium is - and explain in detail?
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.