This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Although tetracycline is a broad-spectrum antibiotic but resistance to the drug developed by the following mechanism: 
Select and explain. 
A. By inactivating enzymes., B. An efflux pump mechanism., C. Alteration of the binding site., D. Absence of a metabolic pathway.
B: An efflux pump mechanism.
Answer: B: An efflux pump mechanism. 
Explanation: Tetracyclines are broad-spectrum bacteriostatic drugs. Development of resistance to  tetracyclines are mainly due to the development of efflux pumps.