Final answer:
Orlistat inhibits the enzyme lipase in the gastrointestinal tract, preventing the absorption of dietary fat.
Step-by-step explanation:
The over-the-counter weight loss medication orlistat (brand name all) inhibits the enzyme lipase in the gastrointestinal tract. Lipase is responsible for breaking down dietary fats into smaller molecules that can be absorbed by the body. By inhibiting this enzyme, orlistat prevents the absorption of some of the dietary fat, leading to weight loss.