microbiology chapter 1 test bank

Correct Answer: B

Rationale: Gas gangrene is caused by Clostridium perfringens due to its ability to produce gas in tissues leading to necrosis. Other choices are incorrect: A (C. difficile causes diarrhea), C (C. tetani causes tetanus), and D (incorrect as B is the causative agent).

Correct Answer: D

Rationale: The correct answer is D: Shigella dysenteriae I. This bacterium is known to produce Shiga toxin, which is responsible for the most severe bacillary infections. The toxin can cause bloody diarrhea and potentially life-threatening complications like hemolytic uremic syndrome. Shigella boydii, flexneri, and sonnei, while also causing bacillary dysentery, do not produce the same level of severity as Shigella dysenteriae I due to the absence of Shiga toxin.

Correct Answer: D

Rationale: The correct answer is D: Viscerotropic Leishmania. The presence of crescent-shaped bodies with pointed tips, blue cytoplasm, and red nuclei is characteristic of Leishmania species. Viscerotropic Leishmania causes visceral leishmaniasis, also known as kala-azar, which affects the internal organs. Toxoplasma typically presents as tachyzoites, not crescent-shaped bodies. Plasmodium malariae is a malaria parasite with different morphology. Dermatotropic Leishmania causes cutaneous leishmaniasis, not the visceral form indicated by the morphology described in the question.

Correct Answer: B

Rationale: The correct answer is B because the capsule of bacteria is mainly composed of polysaccharides or polypeptides. This outer layer helps bacteria evade the host's immune system and protects them from desiccation. Peptidoglycan (choice A) is a component of the bacterial cell wall, not the capsule. Flagellin (choice C) is a protein component of the bacterial flagella, not the capsule. Choice D is incorrect as the capsule is indeed composed of polysaccharides or polypeptides.

Correct Answer: A

Rationale: The correct answer is A: Giardia lamblia. Giardia is a flagellated protozoa with two nuclei, causing diarrhea. It is commonly found in contaminated water sources. Entamoeba histolytica (B) causes amoebic dysentery with trophozoites in stool. Trichomonas vaginalis (C) causes sexually transmitted infections, not diarrhea. Balantidium coli (D) is a ciliated protozoa causing dysentery with ciliated trophozoites. Therefore, Giardia lamblia is the most likely causative agent in this case.