This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Protein Synthesis Inhibitors, Subject name: Pharmacology
Which of the following drug is active selectively against Anaerobic and Microaerophilic organisms ? 
Select and explain. 
A. Clindamycin, B. Tetracycline, C. Metronidazole, D. Aminoglycosides
C: Metronidazole
Answer: C: Metronidazole 
Explanation: Metronidazole undergoes reduction reaction by receiving single electron from Microaerophilic and Anaerobes pathogens which conves into a highly reactive Nitroradical Anion and damages the pathogen DNA . Metronidazole manifests antibacterial activity against all Anaerobic cocci; Anaerobic gram-negative bacilli(incl.Bacteroides sp.); Anaerobic spore-forming gram-positive bacilli (such as Clostridium); and Microaerophilic bacteria such as Helicobacter and Campylobacter sp. So Metronidazole is less active against Aerobic organisms because of the lack of Radical-mediated mechanism. Nonsporulating gram-positive bacilli often are resistant, as they are aerobic and facultatively anaerobic bacteria.