Mr. John, a 45-year-old male, presents to the clinic with complaints of a 3-month history of heartburn and regurgitation, particularly after large meals or lying down. He reports a burning sensation in his chest that worsens after eating spicy or fatty foods. He has been self-medicating with over-the-counter antacids, which provide temporary relief, but his symptoms persist. Medical History Past Medical History: No history of chronic illness. Medications: Occasional use of antacids. Social History: Smokes 1 pack of cigarettes per day; occasional alcohol consumption. Physical Examination Vitals: Blood pressure 130/85 mmHg, heart rate 78 bpm, BMI 28 kg/m². General: Appears well-nourished and in no acute distress. Abdominal Exam: Soft, non-tender abdomen with no organomegaly. Based on clinical presentation, you diagnosed his condition as Gastroesophageal reflux disease (GERD) and started him on famotidine 20 mg twice daily. What is the primary mechanism of action of famotidine
