Which of the following bacteria is capable of surviving in extreme heat?

Correct Answer: B

Rationale: The correct answer is B: Bacillus subtilis. Bacillus subtilis is a thermophilic bacterium capable of surviving in extreme heat due to its heat-resistant spores. It forms spores when exposed to unfavorable conditions, allowing it to withstand high temperatures. Escherichia coli (A), Staphylococcus aureus (C), and Clostridium botulinum (D) are mesophilic bacteria that do not have the same heat resistance mechanisms as Bacillus subtilis, making them unable to survive in extreme heat conditions.

Correct Answer: B

Rationale: The correct answer is B: Plasmids. Plasmids are extrachromosomal DNA molecules found in bacteria that can replicate independently of the bacterial chromosome. They carry additional genetic information, such as antibiotic resistance genes or virulence factors. Mesosomes (choice A) are not involved in carrying genetic information; they are infoldings of the cell membrane. Ribosomes (choice C) are cellular organelles responsible for protein synthesis, not carriers of genetic information. Choice D is incorrect as plasmids are indeed extrachromosomal carriers of genetic information in bacteria.

Correct Answer: B

Rationale: The correct answer is B: placenta. The placenta is a highly specialized structure that forms during pregnancy and serves as a barrier, protecting the fetus from harmful microorganisms that could potentially harm the developing baby. It facilitates the exchange of nutrients, oxygen, and waste between the mother and fetus while preventing the passage of most bacteria and viruses. The uterus (A) is the organ where the fetus develops, but it does not have the specific protective functions of the placenta. The vagina (C) is the birth canal and does not provide protection from microorganisms for the fetus. The mammary gland (D) is responsible for producing milk after birth and is not directly involved in protecting the fetus from microorganisms during pregnancy.

Correct Answer: C

Rationale: Step-by-step rationale for why C is correct: 1. Immunomodulators regulate the immune response. 2. Gamma-interferon is a cytokine that enhances immune function. 3. It helps fight infections and regulate inflammation. 4. Pentaxim and Hexacima are combination vaccines. 5. Rifampin is an antibiotic, not an immunomodulator. Summary: - A and B are combination vaccines. - D is an antibiotic. - C is the correct answer as gamma-interferon is an immunomodulator that regulates the immune response.

Correct Answer: A

Rationale: The correct answer is A: Candida fungi. In the context of a 70-year-old man with angular cheilitis (lesion at the corners of the mouth) and stomatitis, the presence of large ovoid gram-positive cells points towards Candida species, which are yeast fungi. Candida is a common opportunistic pathogen that can cause oral thrush and angular cheilitis in immunocompromised individuals, such as elderly people. Candida fungi are known to appear as large ovoid cells under microscopic examination. Summary of other choices: B: Streptococci are gram-positive bacteria and are not typically associated with large ovoid cells seen in this case. C: Staphylococci are also gram-positive bacteria and are not known to present as large ovoid cells in this context. D: Neisseria are gram-negative bacteria and are not the usual etiological agents for oral lesions like angular cheilitis.

Correct Answer: A

Rationale: The correct answer is A: Blood. Toxoplasmosis is diagnosed through serological tests that detect specific antibodies in the blood. These antibodies indicate exposure to the Toxoplasma gondii parasite. Feces, urine, and duodenal contents are not typically used for diagnosing toxoplasmosis as the parasite is not commonly found in these samples. Blood is the primary material used as it provides the necessary antibodies for accurate diagnosis.

Correct Answer: A

Rationale: The correct answer is A: Neisseria species. Gram staining differentiates bacteria based on cell wall composition. Gram-negative bacteria like Neisseria have a thin peptidoglycan layer and an outer membrane, making them appear pink under the microscope. Streptococcus species (B) are Gram-positive cocci with a thick peptidoglycan layer, appearing purple. Bacillus (C) and Clostridium (D) species are Gram-positive rods, not Gram-negative cocci. Thus, the correct answer is A because Neisseria species are the only Gram-negative cocci among the choices provided.

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus aureus. Staphylococcus aureus is a Gram-positive cocci that is catalase-positive and coagulase-positive, making it the most likely causative agent for the wound infection. Catalase-positive bacteria produce the enzyme catalase, which helps break down hydrogen peroxide. Coagulase-positive bacteria produce the enzyme coagulase, which causes the plasma proteins to clot. Staphylococcus epidermidis is catalase-positive but coagulase-negative, making it less likely to cause a wound infection. Streptococcus pyogenes is catalase-negative, and Enterococcus faecalis is catalase-negative and coagulase-negative, so they are less likely to be the causative agent.

Correct Answer: A

Rationale: The correct answer is A: Acid-fast stain. This stain is used for diagnosing tuberculosis due to the unique cell wall composition of Mycobacterium tuberculosis, which resists decolorization by acid-alcohol. This allows the bacteria to retain the primary stain (carbol fuchsin) and appear red under a microscope. Gram stain (B) is not suitable for Mycobacterium tuberculosis as it is gram-positive but does not retain the crystal violet-iodine complex. Negative stain (C) is not used for bacterial identification. Simple stain (D) does not differentiate acid-fast bacteria like Mycobacterium tuberculosis.