A 52-year-old man presents to the clinic with complaints of persistent fatigue, generalized weakness, and occasional episodes of confusion over the past few months. He also reports frequent urination and excessive thirst, which he initially attributed to aging. Over the last several weeks, he has developed intermittent abdominal pain, particularly after meals, and occasional nausea. He reports severe constipation. He denies recent weight loss, but notes difficulty concentrating and frequent headaches. His family history is significant for a brother who was diagnosed with a “parathyroid tumor” in his 40s and a father who passed away at an early age due to complications of “endocrine issues.” On physical examination, the patient appears mildly dehydrated but otherwise stable. His blood pressure is 140/90 mmHg, and his pulse is regular at 82 beats per minute. There are no overt abnormalities on inspection or palpation of the neck. Cardiovascular and respiratory examinations are unremarkable. An abdominal examination reveals mild epigastric tenderness without masses or organomegaly. Laboratory studies show a serum calcium level of 12.8 mg/dL (normal range: 8.5–10.5 mg/dL), and a serum phosphate level of 2.0 mg/dL (normal range: 2.5–4.5 mg/dL). Additional workup reveals elevated gastrin levels, prompting further investigation with an upper endoscopy, which identifies multiple duodenal ulcers. Genetic testing demonstrated a mutation in a gene, which encodes menin, a tumor suppressor protein. What is the most likely diagnosis in this patient?
