The absorption of tetracyclines is decreased by which of the following?

Tetracyclines are a class of antibiotics that are known to have their absorption significantly affected by the presence of cations, which include ions such as calcium, magnesium, and iron. These cations can bind to tetracyclines in the gastrointestinal tract, forming insoluble complexes that inhibit the drug's ability to be absorbed into the bloodstream.

When tetracyclines are taken simultaneously with mineral supplements or foods high in cations (like dairy products), the effectiveness of the medication can be compromised due to this interaction. Therefore, it is essential to avoid taking tetracyclines with such substances to ensure proper absorption and efficacy of the antibiotic treatment.

The other options do not have the same pronounced effect on tetracycline absorption. Vitamins may have variable interactions, but they are not known to consistently decrease absorption like cations do. Proteins and fats do not typically form complexes with tetracyclines that affect their absorption to the same extent as cations.