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: Surgery
A 52 year old man presented with dysphagia diagnosed of having adenocarcinoma of esophagus. It mostly seen in: 
Select and explain. 
A. Middle esophagus, B. Upper esophagus, C. Barrett's ulcer, D. None of the above
C: Barrett's ulcer
Answer: C: Barrett's ulcer 
Explanation: Barrett esophagus secondary to long-standing reflux disease is the single most impoant risk factor for adenocarcinoma. Patients with Barrett esophagus are 30 to 125 times more likely to have adenocarcinoma of the esophagus than the general population. Microscopically, adenocarcinoma almost always originates in metaplastic Barrett's mucosa, and resembles gastric cancer. Ref: Jobe B.A., Hunter J.G., Peters J.H. (2010). Chapter 25. Esophagus and Diaphragmatic Hernia. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.