A fecal sample cultured on Endo agar revealed colorless colonies. The bacteria were Gram-negative rods and lactose non-fermenters. What microorganism is most likely?

Correct Answer: C

Rationale: The correct answer is Shigella (Choice C) because it matches the description of being Gram-negative rods and lactose non-fermenters. Shigella is a non-lactose fermenting, Gram-negative rod that typically produces colorless colonies on Endo agar. Salmonella (Choice A) and Escherichia coli (Choice B) are both lactose fermenters and would produce pink colonies on Endo agar. Proteus (Choice D) is also a lactose fermenter and would appear pink on Endo agar. Therefore, based on the characteristics provided, Shigella is the most likely microorganism in this scenario.

Correct Answer: B

Rationale: The correct answer is B: Syphilis. The Wasserman reaction is a test for syphilis, specifically for detecting antibodies against the bacterium Treponema pallidum. In a 30-year-old man with a markedly positive Wasserman reaction, it indicates an active syphilis infection. Brucellosis (Choice A) is not diagnosed using the Wasserman reaction. Poliomyelitis (Choice C) is a viral infection affecting the spinal cord, not diagnosed using the Wasserman reaction. Influenza (Choice D) is a respiratory viral infection, not associated with the Wasserman reaction.

Correct Answer: C

Rationale: The correct answer is Shigella (Choice C) because it matches the description of being Gram-negative rods and lactose non-fermenters. Shigella is a non-lactose fermenting, Gram-negative rod that typically produces colorless colonies on Endo agar. Salmonella (Choice A) and Escherichia coli (Choice B) are both lactose fermenters and would produce pink colonies on Endo agar. Proteus (Choice D) is also a lactose fermenter and would appear pink on Endo agar. Therefore, based on the characteristics provided, Shigella is the most likely microorganism in this scenario.

Correct Answer: C

Rationale: The correct answer is C: Flaviviridae. Dengue virus belongs to the Flaviviridae family because it is a flavivirus. Flaviviruses are a group of RNA viruses known to cause diseases in humans, such as dengue fever, Zika virus, and yellow fever. This family is characterized by its single-stranded, positive-sense RNA genome and enveloped virion structure. Hepadnaviridae (choice A), Parvoviridae (choice B), and Picornaviridae (choice D) are all different families of viruses that do not include the dengue virus. Hepadnaviridae includes hepatitis B virus, Parvoviridae includes parvoviruses, and Picornaviridae includes enteroviruses like poliovirus.

Correct Answer: B

Rationale: Step-by-step rationale for why choice B (Tuberculin is introduced) is correct: 1. Tuberculin test helps identify individuals with latent TB infection. 2. Negative skin-allergic samples suggest absence of active TB. 3. Introducing tuberculin can help confirm latent TB infection. 4. Antibiotics are not prescribed for negative skin-allergic samples. 5. Vaccines are not administered for TB diagnosis. Summary: - Choice A is incorrect as antibiotics are not prescribed for negative skin-allergic samples. - Choice C is incorrect as vaccines are not used for TB diagnosis. - Choice D is incorrect as introducing tuberculin is the appropriate step in this scenario.

Correct Answer: A

Rationale: The correct answer is A: Head lice. Epidemic typhus is transmitted by the body louse, Pediculus humanus humanus, which is different from the head louse, Pediculus humanus capitis. Head lice are not known to transmit epidemic typhus. Body lice are the primary vectors for transmitting epidemic typhus. Fleas are vectors for other diseases such as plague, not epidemic typhus. Ticks are vectors for various diseases but not for epidemic typhus. Therefore, the correct vector for epidemic typhus transmission is body lice, making choice A the correct answer.

Correct Answer: B

Rationale: Rationale: The correct answer is B because virulence factors of bacteria are characteristics that enable them to cause disease. Colonization allows bacteria to establish and multiply in the host. Invasion refers to the ability to penetrate host tissues. Toxins are substances produced by bacteria that harm the host. Adhesion is crucial for bacteria to attach to host cells. A: Size and mutation are not typical virulence factors. Resistance is not directly related to causing disease. C: While toxins and evasion of body defenses are correct, motility and size are not universally considered virulence factors. D: Chemotaxis and fermentation are not primary virulence factors, unlike colonization and toxins.

Correct Answer: A

Rationale: Ribosomes are the target for antibiotics as they interfere with protein synthesis in bacteria. Aminoglycosides, tetracyclines, macrolides, and lincosamides all target ribosomes by inhibiting protein synthesis at different stages. Beta-lactam antibiotics target cell wall synthesis, not ribosomes. Antifolate compounds, quinolones, and raspicin target DNA synthesis and replication, not ribosomes. Therefore, the correct answer is A as it includes antibiotics that specifically target ribosomes for inhibiting bacterial protein synthesis.

Correct Answer: B

Rationale: The correct answer is B. Immunity after a single tetanus vaccination is of limited duration, usually around 10 years. Booster vaccinations are needed to maintain protection due to waning immunity over time. Choice A is incorrect as immunity is not usually lifelong. Choice C is incorrect as tetanus toxoid is a sterile vaccine and does not cause infection. Choice D is also incorrect as there is a correct answer, which is B.