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Guaiac-based fecal occult blood tests (gFOBT) detect the peroxidase activity of heme. Certain foods and medications can cause false positives or negatives. Patients are typically instructed to avoid red meat (which contains peroxidase activity) and high-dose vitamin C (which can cause false negatives by inhibiting the color reaction) for about three days before and during the collection period. Option B aligns with standard guidance. Encouraging red meat (Option A) is the opposite of recommended practice. Aspirin/NSAIDs (Option C) may increase gastrointestinal bleeding risk and potentially cause false positives; many protocols advise avoiding them if medically permissible. Collecting during menstruation (Option D) risks contamination with menstrual blood and invalid results. Additional instructions include avoiding certain raw vegetables with peroxidase activity (e.g., turnips, horseradish), following kit directions for multiple samples on separate days, and promptly returning the cards. Clear counseling improves test specificity and reduces unnecessary follow-up procedures—competencies emphasized for phlebotomy professionals involved in patient instruction.