Q. A 45-year-old man presents to his primary care physician with a six-month history of abdominal pain, nausea, and vomiting. He reports that these symptoms have been progressively worsening and that he has been feeling more tired than usual. He also reports a history of peptic ulcer disease, which was diagnosed ten years ago. The patient's vital signs on presentation are: blood pressure 120/80 mm Hg, pulse rate 80 beats/minute, and respiratory rate 16 breaths/minute. The physical examination reveals a patient with abdominal tenderness and a history of peptic ulcer disease. Further evaluation reveals elevated levels of gastrin in the patient's blood and an abnormal growth in the duodenum on endoscopy. The patient is diagnosed with a gastrinoma, a type of neuroendocrine tumor that secretes excessive amounts of gastrin and causes hypersecretion of gastric acid. What is the signaling pathway of gastrin?