<prompt> The most common underlying aetiology of primary hyperparathyroidism isParathyroid adenomaMEN-1MEN-2AParathyroid adenocarcinoma  <response>: The underlying aetiology of PHPT is usually a solitary parathyroid adenoma, however, in a small number of patient 2-4, there are double adenomas. It may occur in a sporadic fashion or it can be familial MEN type 1 or type 2A, hyperparathyroidism-jaw tumour syndrome HPT-JTRef: Bailey and Love 27e pg: 826 