Antacids, PPIs, H2 Blockers

Which H2 receptor antagonist has the longest half-life and allows once-daily dosing?

A 52-year-old man with a 10-year history of GERD presents with persistent epigastric pain despite 8 weeks of omeprazole 40 mg daily. Upper endoscopy reveals a 2 cm ulcer in the gastric antrum with surrounding erythema. Serum gastrin level is 850 pg/mL (normal <100). Which investigation is most appropriate to confirm the diagnosis of Zollinger–Ellison syndrome?

Which feature best distinguishes proton pump inhibitors (PPIs) from H2-receptor antagonists in terms of their mechanism and clinical efficacy?

A 52-year-old man from Delhi presents with a 6-month history of epigastric pain, heartburn, and regurgitation. He has been self-medicating with sodium bicarbonate antacid 4–5 times daily. On examination, he appears well, BP 128/82 mmHg, HR 88/min. Serum sodium is 148 mEq/L (normal 135–145), serum chloride is 92 mEq/L (normal 96–106), and serum bicarbonate is 32 mEq/L (normal 22–26). Arterial pH is 7.48. What is the most likely acid–base disturbance and the primary mechanism underlying his electrolyte abnormality?

A 48-year-old woman on chronic omeprazole therapy for GERD presents with progressive fatigue, glossitis, and paresthesias in the lower limbs. Serum vitamin B12 level is 180 pg/mL (normal 200–900). Which investigation is most appropriate to confirm the mechanism of B12 deficiency in this patient?