Introduction:
An obligate anaerobe is an organism that requires an oxygen-free environment to survive and cannot tolerate the presence of oxygen. These organisms obtain energy through anaerobic respiration or fermentation. They are unable to use oxygen as a terminal electron acceptor during respiration and may even be harmed or killed by exposure to oxygen.

Explanation:
The correct answer to the question is option B, Clostridium tetani. Clostridium tetani is a bacterium that causes tetanus, a severe neurological disease. It is an obligate anaerobe, which means it cannot grow in the presence of oxygen.

Clostridium tetani:
Clostridium tetani is a Gram-positive, spore-forming bacterium that is commonly found in soil and feces. It can also be found in contaminated wounds. The bacterium produces a neurotoxin called tetanospasmin, which is responsible for the symptoms of tetanus.

Obligate Anaerobe:
As an obligate anaerobe, Clostridium tetani cannot survive in the presence of oxygen. Oxygen is toxic to the bacterium and can inhibit its growth. This is because it lacks the necessary enzymes to neutralize the harmful effects of oxygen.

Survival and metabolism:
Clostridium tetani obtains energy through anaerobic respiration or fermentation. It uses alternative electron acceptors such as nitrate or sulfate in the absence of oxygen. The bacterium can produce ATP through glycolysis and fermentation pathways.

Tetanus infection:
Tetanus infection occurs when the spores of Clostridium tetani enter the body through a wound or injury. The spores germinate in an anaerobic environment and release the neurotoxin. The neurotoxin travels through the bloodstream and affects the nervous system, leading to muscle stiffness, spasms, and potentially life-threatening complications.

Treatment and prevention:
The treatment for tetanus involves wound care, administration of tetanus antitoxin, muscle relaxants, and antibiotics. Prevention is achieved through vaccination with the tetanus toxoid vaccine, which provides immunity against the neurotoxin.

In conclusion, Clostridium tetani is an obligate anaerobe that cannot survive in the presence of oxygen. It causes tetanus, a severe neurological disease, and obtains energy through anaerobic respiration or fermentation.