A 70-year-old female patient complains of burning sensation under the base of her complete denture; dryness and bitterness in the mouth. The denture was fabricated 3 weeks ago. Objectively: the alveolar bone mucosa is edematous, there is diffuse hyperemia up to the margins of the denture. The tongue is hyperemic, dry and cracked. What additional study would be most appropriate in this case?