Fluorosis (dental and skeletal) is caused by an excessive concentration of fluoride ions in drinking water.

Introduction / Context:
Fluorosis is a public health issue arising from prolonged ingestion of high levels of fluoride in drinking water. It affects teeth (dental fluorosis) and bones (skeletal fluorosis).

Given Data / Assumptions:

• Problem concerns the specific contaminant responsible for fluorosis.
• Other heavy metals cause different toxicities.

Concept / Approach:
Associate each contaminant with its characteristic health effect and identify which one uniquely causes fluorosis.

Step-by-Step Solution:

Verification / Alternative check:
Public health guidelines set fluoride limits; areas with geogenic fluoride show higher fluorosis prevalence.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing fluoride with arsenic issues; assuming any heavy metal causes fluorosis; ignoring exposure duration.

Final Answer:
Fluoride