A 44-year-old woman has had increasing abdominal distention for the past 6 weeks. On physical examination, there is an abdominal fluid wave, and bowel sounds are present. Paracentesis yields 1000 mL of slightly cloudy serous fluid. Cytologic examination of the fluid shows malignant cells consistent with adenocarcinoma. Molecular analysis of these cells shows an MSH2 gene mutation with microsatellite instability. Her medical history indicates that she has had no major medical illnesses and no surgical procedures. Her sister was diagnosed with endometrial cancer and her brother had carcinoma of the stomach. Which of the following conditions is the most likely cause of this patient's symptoms?
High-Yield Explanation
Of the conditions listed, the one most likely to lead to adenocarcinoma in a patient of this age is hereditary nonpolyposis colorectal cancer or Lynch syndrome. Crohn disease is unlikely because the patient has not had a prior serious illness, and Crohn's disease of long duration is unlikely to remain asymptomatic. Although adenocarcinoma may complicate Crohn disease, it does not occur as frequently as in ulcerative colitis. This explains why colectomy is often performed for ulcerative colitis, but bowel resections are avoided, if possible, in Crohn disease. The other conditions listed are not premalignant.