During the oral cavity sanation on the vestibular surface of the 21 and 12 teeth in the cervical area there were detected chalky spots. Enamel surface is dull and coarse, no reaction to thermal stimuli. DecayedMissing-Filled Index (dmft/DMFT) is 6, Hygiene Index is 2. Mesial occlusion is observed. Within the first year of life the patient had been suffering from frequent cases of URTI and a case of chicken pox. What additional investigation methods would be useful for the diagnosis-making in this case?