All of the following can contribute to SYSTEMIC fluorosis EXCEPT:

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

• A. Reconstituting powdered infant formula with fluoridated water IS a recognised risk, because the infant's entire fluid intake then carries fluoride during a period of active enamel formation. Ready-to-feed formula or low-fluoride water is advised for exclusively formula-fed infants.
• B. CORRECT — this is the EXCEPTION. Professionally applied TOPICAL fluoride acts at the enamel surface post-eruptively. The quantity of fluoride swallowed from a properly applied varnish is small and the exposure is brief and infrequent, so it does not produce the sustained systemic elevation required for fluorosis. Its mechanism is surface remineralisation and formation of calcium fluoride reservoirs, not incorporation into developing enamel.
• C. Swallowing toothpaste IS a leading cause of fluorosis in developed countries. Young children lack a mature swallowing reflex and readily ingest paste, which is why a smear (rice-grain) is used under age 3 and a pea-sized amount from 3 to 6, always with supervision.
• D. Drinking water is the classic systemic source. The optimal concentration is 0.7 ppm; sustained exposure above that during enamel formation produces fluorosis, and endemic fluorosis in high-fluoride regions is the historical basis of the entire literature.

Note: The whole concept turns on TIMING: fluorosis is a PRE-ERUPTIVE, developmental defect caused by systemic fluoride during enamel formation — the critical window for the permanent incisors and first molars being roughly the first three years of life. Once a tooth has erupted, it can no longer become fluorosed.