This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anti Microbial, Subject name: Pharmacology
Which one of the following is a mechanism underlying the resistance of strains of S. pneumoniae to the widely used antibiotic, ciprofloxacin? 
Select and explain. 
A. Reduced topoisomerase sensitivity to inhibitors, B. Increased synthesis of PABA, C. Formation of methyltransferases that change receptor structure, D. Structural changes in porins
A: Reduced topoisomerase sensitivity to inhibitors
Answer: A: Reduced topoisomerase sensitivity to inhibitors 
Explanation: Microbial resistance to fluoroquinolones is increasing, and some strains of Streptococcus pneumoniae are now resistant to ciprofloxacin. The mechanism can involve changes in the structure of topoisomerase IV, one of the "targets" of fluoroquinolones, which inhibit nucleic acid synthesis. Pneumococcal resistance to penicillins is also increasing via changes in penicillin-binding proteins (PBPs). The other mechanisms listed underlie microbial resistance to other antibiotics as follows: sulfonamides (choice B), macrolides (choice C), extended-spectrum penicillins (choice D), and beta-lactams (choice E).