Correct Answer: B

Rationale: The correct answer is B: Vibrio cholerae. Cholera is caused by Vibrio cholerae, a bacterium that infects the small intestine, leading to severe diarrhea and dehydration. This bacterium produces a toxin that causes the symptoms of cholera.
A: Shigella dysenteriae causes dysentery, not cholera.
C: Salmonella enterica causes food poisoning, not cholera.
D: Escherichia coli can cause food poisoning but not cholera.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pyogenes.
1. Gram-positive cocci in chains: characteristic of Streptococcus species.
2. Catalase-negative: Streptococcus species are catalase-negative.
3. Beta-hemolytic: S. pyogenes is beta-hemolytic on blood agar.

Therefore, S. pyogenes fits all these criteria.
B: Streptococcus pneumoniae is catalase-negative but alpha-hemolytic, not beta-hemolytic.
C: Enterococcus faecalis is catalase-negative, but not typically beta-hemolytic or in chains.
D: Staphylococcus aureus is catalase-positive and not in chains.

Correct Answer: B

Rationale: The correct answer is B: Salmonella enteritidis. Salmonellosis is typically caused by Salmonella species, with Salmonella enteritidis being one of the common culprits. Salmonella typhi causes typhoid fever, not salmonellosis. Shigella flexneri is responsible for causing shigellosis, a different bacterial infection. Choosing "None of the above" is incorrect as Salmonella enteritidis is a known cause of salmonellosis.

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:
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Choice A is incorrect as antibiotics are not prescribed for negative skin-allergic samples.
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Choice C is incorrect as vaccines are not used for TB diagnosis.
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Choice D is incorrect as introducing tuberculin is the appropriate step in this scenario.

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.