The correct answer is Tetracycline.

- Tetracycline's gastrointestinal absorption is significantly reduced when taken concurrently with dairy products due to a phenomenon called chelation.
- Dairy products contain divalent and trivalent cations such as calcium, magnesium, and iron.
- These cations can bind to tetracycline molecules in the gastrointestinal tract, forming an insoluble complex that prevents the antibiotic from being absorbed into the bloodstream effectively.
- This interaction results in decreased bioavailability and reduced therapeutic efficacy of tetracycline.

In contrast, antibiotics like ciprofloxacin can also chelate with cations but tetracycline is the classic example widely known for this interaction. Macrolides such as clarithromycin and azithromycin do not undergo chelation with dairy products and hence do not have significant reduction in absorption when taken with dairy.

Important Points:
- Tetracycline binds with divalent/trivalent cations (Ca²⁺, Mg²⁺, Fe³⁺) present in dairy.
- Chelation leads to formation of insoluble complexes.
- These complexes reduce the gastrointestinal absorption and bioavailability.
- Administer tetracycline at least 1-2 hours before or after dairy intake to avoid this issue.
- Macrolides (clarithromycin, azithromycin) and fluoroquinolones (ciprofloxacin) have different interaction profiles.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 39: Antimicrobial Drugs / Page 1234