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
The most common cause of severe hypercalcemia is- 
Select and explain. 
A. Vitamin D toxicity, B. Sarcoidosis, C. Chronic renal failure, D. Malignancy
D: Malignancy
Answer: D: Malignancy 
Explanation: Causes of hypercalcaemia are listed in Box 20.36. Of these, primary hyperparathyroidism and malignant hypercalcaemia are by far the most common. Familial hypocalciuric hypercalcaemia (FHH) is a rare but impoant cause that needs differentiation from primary hyperparathyroidism (HPT). Lithium may cause hyperparathyroidism by reducing the sensitivity of the calcium-sensing receptor. Causes of hypercalcaemia- With normal or elevated PTH levels *Primaryoeiaryhyperparathyroidism *Lithium-inducedhyperparathyroidism *Familialhypocalciuric hypercalcaemia With low PTH levels *Malignancy(lung,breast,myeloma,renal,lymphoma, thyroid) *Elevated1,25(OH)2vitaminD(vitaminDintoxication, sarcoidosis,HIV,othergranulomatousdisease) *Thyrotoxicosis *Paget'sdiseasewithimmobilisation *Milk-alkalisyndrome *Thiazidediuretics *Glucocoicoiddeficiency DAVIDSONS PRINCIPLE AND PRACTICE OF MEDICINE 22ND EDITION PAGENO-767