Dental fluorosis can be caused by all of the following EXCEPT:

Correct answer: C. Professionally applied topical fluoride in the dental office

• A. IS a cause, and a major one. Young children swallow a substantial proportion of dentifrice, which is why a smear (rice grain) is advised under 3 years and a pea-sized amount from 3 to 6.
• B. IS a cause — of very mild forms. Optimally fluoridated water at 0.7 ppm produces a low prevalence of very mild fluorosis, an accepted trade-off for the caries reduction. Above-optimal or naturally high-fluoride water causes more severe forms.
• C. CORRECT — this is the EXCEPTION. Professionally applied TOPICAL fluoride (varnish, gel, foam) is delivered in a supervised setting with suction and controlled quantity, and it acts on the erupted tooth SURFACE. Fluorosis is a DEVELOPMENTAL defect requiring chronic SYSTEMIC ingestion during amelogenesis, so a controlled topical application does not cause it.
• D. IS a cause. Powdered or concentrated formula reconstituted with fluoridated water is one of the recognised sources of excess systemic fluoride in infancy, which is why the ADA advises occasional use of low-fluoride water for exclusively formula-fed infants.

Note: Please correct in the group file. Fluorosis is only possible while enamel is forming — the critical window for permanent incisors closes at about age 6–8.