This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A gastric ulcer is said to be refractory when it fails to heal after therapy for: 
Select and explain. 
A. 6 weeks, B. 8 weeks, C. 12 weeks, D. 14 weeks
C: 12 weeks
Answer: C: 12 weeks 
Explanation: A gastric ulcer that fails to heal after 12 weeks and a duodenal ulcer does not heal at 8 weeks of therapy should be considered refractory. Cause of refractory ulcers: Malignancy Zollinger Ellison syndrome Cigarette smoking Ischemia Crohn's disease Lymphoma Eosinophilic gastroenteritis Infections (TB,CMV) Ref: Harrisons principles of internal medicine, 18th edition, Page: 2452