Which of the following organisms does not utilize oxygen regardless of its availability?

• A. Facultative anaerobe
• B. Obligate aerobe
• C. Obligate anaerobe
• D. Microaerophile

Answer: (C)

Verified step by step guidance

1. Understand the definitions of each type of organism in relation to oxygen utilization: Facultative anaerobes can use oxygen but can also survive without it; Obligate aerobes require oxygen to survive; Microaerophiles require oxygen but at lower concentrations than atmospheric levels; Obligate anaerobes cannot tolerate oxygen and do not use it regardless of its availability.
2. Identify the key phrase in the question: 'does not utilize oxygen regardless of its availability,' which means the organism neither uses oxygen nor can survive in its presence.
3. Match this characteristic to the definitions: Facultative anaerobes and microaerophiles both use oxygen to some extent, and obligate aerobes require oxygen, so they do not fit the description.
4. Recognize that obligate anaerobes are the organisms that do not use oxygen at all and are often harmed by its presence, fitting the question's criteria perfectly.
5. Conclude that the correct answer is obligate anaerobe because it neither utilizes oxygen nor survives in its presence, unlike the other options.