C. Measure serum niacin level
The key to the correct diagnosis in this patient is the presence of persistent nonbloody diarrhea, rash in sun-exposed areas, and glossitis in an older frail woman with severe malnutrition—features key to diagnosing pellagra, which is characterized by 4 Ds (dermatitis, diarrhea, dementia, and death) if left untreated.1
Measurement of fecal calprotectin level (choice A) is not indicated in this patient with no findings of inflammatory bowel disease such as fever, abdominal pain, bloody diarrhea, or elevated levels of inflammatory markers. Because skin changes in sun-exposed areas and glossitis are atypical of parasitic infection and this patient had no eosinophilia, checking for fecal ova and parasites (choice B) also is not indicated. Given the patient’s symptoms suggestive of pellagra, measurement of serum niacin level (choice C) would be appropriate. Testing for serum antitissue transglutaminase antibody (choice D), a screening test for celiac disease, is not immediately indicated in this patient, as her rash is not consistent with dermatitis herpetiformis.