A 45 years old male presents with abdominal pain, diarrhea, and heartburn. Physical exam is unremarkable and medical history is notable only for a past episode of nephrolithiasis. On endoscopy he is found to have multiple gastric and duodenal ulcers. The patient is prescribed proton pump inhibitors and treated for Helicobater pylori, which relieves his symptoms temporarily. However, he has multiple recurrences of symptoms and ulcers. What is the most likely diagnosis?