The correct answer is C, the patient has just been given a barium enema.  Stools collected immediately following a barium enema are unsuitable for laboratory examination.  Keeping a specimen overnight in the refrigerator (answer A) is not a reason to reject a specimen for parasitology examination.  Parasites keep their morphological integrity if the specimen is kept refrigerated for up to 48 hours when preservatives are not available.  Diarrheic stools (answer B) and stools from patients with no symptoms of infections (answer D) are commonly analyzed with valid results obtained in diagnostic laboratories.