Correct Answer: D

Rationale: Step-by-step rationale: 1. Treponema pallidum and Borrelia burgdorferi are both spirochetes. 2. Spirochetes are characterized by their spiral shape and unique motility. 3. Both organisms exhibit corkscrew-like movement due to axial filaments. 4. Other choices are incorrect as Treponema and Borrelia are not gram-negative rods, cocci, or anaerobic cocci. Summary: The correct answer is D because Treponema pallidum and Borrelia burgdorferi are spirochetes, characterized by their spiral shape and motility. Other choices are incorrect as they do not accurately describe the characteristics of these organisms.

Correct Answer: A

Rationale: The correct answer is A: Trichomonas vaginalis. Trichomonas vaginalis is a protozoan parasite that causes trichomoniasis, a sexually transmitted infection characterized by inflammation of urogenital tracts. The description of big unicellular pyriform organisms with a sharp spike on the back end of their bodies, big nucleus, and undulating membrane matches the morphology of Trichomonas vaginalis. Trichomonas hominis and Trichomonas buccalis are different species that do not cause urogenital infections. Trypanosoma gambiense is a protozoan parasite causing African trypanosomiasis and does not match the described morphology or symptoms.

Correct Answer: B

Rationale: The correct answer is B: non-spore forming encapsulated gram-negative bacterium. Francisella tularensis is a non-spore forming bacterium, which rules out choices A and D. It is encapsulated, which eliminates choice C. The encapsulation of the bacterium helps it evade the host immune system, leading to its pathogenicity. This encapsulated bacterium is responsible for causing the zoonotic disease tularemia in humans.

Correct Answer: A

Rationale: The correct answer is A: protozoans. Pinworms are commonly referred to as Enterobius vermicularis, which are parasitic worms belonging to the phylum Nematoda. Protozoans are single-celled organisms, and pinworms are multicellular worms. Algae are photosynthetic organisms, helminths are parasitic worms (which pinworms fall under), and fungi are eukaryotic organisms that include molds and yeasts. Therefore, protozoans are the correct choice for organisms commonly referred to as pinworms.

Correct Answer: D

Rationale: Step-by-step rationale for why answer choice D is correct: 1. Coronaviruses can cause reinfections due to potential waning immunity. 2. Studies have shown cases of reinfections with different strains or within a short period. 3. This contradicts the notion that reinfections cannot be observed. Summary: A: Incorrect because some coronaviruses can cause lower respiratory tract infections. B: Incorrect as coronaviruses can exhibit varied symptomatology including asymptomatic cases. C: Incorrect as some coronaviruses like SARS can lead to severe pneumonia and complications.