A 32-year-old woman presents to the office and reports that she feels unwell and tired.

This patient has microcytic anemia (MCV 75 fL) with very low ferritin , confirming iron deficiency anemia. In a young woman with chronic diarrhea and abdominal discomfort, MCCQE objectives emphasize investigating for malabsorption syndromes , particularly celiac disease . Iron deficiency may be the only presenting feature of celiac disease due to impaired absorption in the proximal small intestine (duodenum), where iron uptake normally occurs.

The most appropriate diagnostic test is IgA tissue transglutaminase (tTG) antibody , which is the recommended first-line serologic test for celiac disease. If positive, diagnosis is typically confirmed with small bowel biopsy. Total iron-binding capacity would only further characterize iron deficiency but would not identify the underlying cause. Fecal fat testing evaluates steatorrhea but is less specific and not first-line for suspected celiac disease. Stool cultures are indicated for acute infectious diarrhea. Helicobacter pylori infection may contribute to anemia but does not explain chronic malabsorptive symptoms.

Thus, IgA tTG testing is most likely to establish the underlying diagnosis.