MRCP2-1644

A 65-year-old man presents for a routine medical check-up at his GP clinic. He has a history of childhood asthma and osteoarthritis in his fingers but is otherwise healthy. During the check-up, his vital signs were recorded as a blood pressure of 129/80 mmHg, pulse rate of 82 bpm, and oxygen saturation of 97%. Blood tests were ordered and the results showed normal levels for most parameters, except for a slightly elevated γGT and serum corrected calcium. A 24-hour urinary calcium test was performed, which revealed a significantly low result. Based on these findings, what is the most likely diagnosis?