The correct answer is B. Irritable bowel syndrome is least likely.
The multiple choices were: A. Lactose intolerance, B. Irritable bowel syndrome, C. Ovarian cyst, D.Gluten enteropathy, E. Inflammatory bowel disease
Explanation: There are many possible causes for recurring low abdominal pain in young adults, some of which appear in the multiple choices. The differential diagnosis includes disorders of the GU, reproductive, and GI systems.
According to the Rome 3 criteria, irritable bowel syndrome (IBS) is defined by recurring abdominal pain at least 3 days per month starting at least 6 months ago associated with at least 2 of the following: a) improvement with defecation; b) onset associated with changes in the frequency of stool; or c) onset associated with changes in the form (appearance) of stool. In addition there should be no “red flags” (such as weight loss or fever) in the history or physical examination.
In the present case, factors that weigh against a diagnosis of IBS are the onset of pain less than 6 months ago, the absence of improvement with defecation or initial changes in the frequency or form of the stool, and the presence of “red flags" such as nocturnal pain, prior travel, menstrual irregularity, and weight loss. All of the remaining diagnoses are possible and require further investigation. (Note that some patients with IBS have "red flags" that prove to be related to other diagnoses. A simple example would be a patient with IBS with rectal bleeding due to a hemorrhoid).
Bottom line: This patient might eventually prove to have IBS, but the lack of criteria and the presence of red flags demands investigation for other causes of her symptoms.