<prompt> The most common cause of severe hypercalcemia is-Vitamin D toxicitySarcoidosisChronic renal failureMalignancy  <response>: 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 Malignancylung,breast,myeloma,renal,lymphoma, thyroid Elevated1,25OH2vitaminDvitaminDintoxication, sarcoidosis,HIV,othergranulomatousdisease Thyrotoxicosis Pagetsdiseasewithimmobilisation Milk-alkalisyndrome Thiazidediuretics Glucocoicoiddeficiency DAVIDSONS PRINCIPLE AND PRACTICE OF MEDICINE 22ND EDITION PAGENO-767 