Correct Answer: A

Rationale: The correct answer is A: Mycobacterium leprae. This bacterium is responsible for causing leprosy because it has a unique ability to infect the skin, nerves, and mucous membranes. It is an acid-fast bacillus that multiplies very slowly, leading to a chronic infection. Clostridium botulinum (choice C) causes botulism, Escherichia coli (choice B) causes various gastrointestinal infections, and Staphylococcus aureus (choice D) causes skin infections and other diseases, but none of these bacteria are associated with leprosy.

Correct Answer: C

Rationale: Step-by-step rationale for why choice C (Nystatin) is the correct answer: 1. Nystatin is an antifungal medication specifically effective against Candida infections. 2. Candida is a type of fungus, not a bacteria targeted by penicillins (choice A) or cephalosporins (choice B). 3. Nystatin works by disrupting the fungal cell membrane, leading to its death. 4. Choice D (None of the above) is incorrect as Nystatin is a suitable treatment option for Candida infections.

Correct Answer: A

Rationale: The correct answer is A: Trichomonas vaginalis. Trichomonas vaginalis is a flagellated protozoan parasite commonly found in vaginal infections. The presence of flagellated bacteria in the vaginal discharge stained with Romanovsky-Giemsa method indicates a protozoan infection, ruling out bacterial or viral causes. Choices B, C, and D are not correct as Leishmania donovani causes leishmaniasis, Trypanosoma gambiense causes African sleeping sickness, and Trihomonas hominis is a non-pathogenic flagellate found in the human intestine, not typically associated with vaginal infections.

Correct Answer: B

Rationale: Binary fission is a form of asexual reproduction where a single organism divides into two equal parts. Bacteria reproduce through binary fission, splitting into two identical daughter cells. Viruses are not considered living organisms and reproduce by infecting host cells. Fungi reproduce through spores or sexual reproduction. Helminths are parasitic worms that reproduce sexually. Therefore, the correct answer is B, bacteria, as they specifically utilize binary fission for reproduction.

Correct Answer: A

Rationale: The correct answer is A: Mycobacterium leprae. This bacterium is responsible for causing leprosy because it has a unique ability to infect the skin, nerves, and mucous membranes. It is an acid-fast bacillus that multiplies very slowly, leading to a chronic infection. Clostridium botulinum (choice C) causes botulism, Escherichia coli (choice B) causes various gastrointestinal infections, and Staphylococcus aureus (choice D) causes skin infections and other diseases, but none of these bacteria are associated with leprosy.