A patient with fever and diarrhea had a stool sample that revealed Gram-negative rods that fermented lactose. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Escherichia coli. E. coli is a Gram-negative rod that ferments lactose. It is a common cause of gastroenteritis presenting with fever and diarrhea. Salmonella typhi does not ferment lactose and causes typhoid fever. Shigella dysenteriae causes dysentery and does not ferment lactose. Proteus vulgaris does not typically cause gastroenteritis and is not known for lactose fermentation.

Correct Answer: A

Rationale: The correct answer is A: Complex. Poxviruses have a complex nucleocapsid symmetry due to their unique structure with multiple layers of proteins and genetic material. Isohedral symmetry (B) refers to a regular polyhedron shape, which is not characteristic of poxviruses. Helical symmetry (C) is found in viruses like tobacco mosaic virus, not poxviruses. "All of the above" (D) is incorrect because poxviruses do not exhibit isohedral or helical symmetry.

Correct Answer: A

Rationale: Correct Answer: A Rationale: 1. Enteropathogenic E. coli (EPEC) attach to enterocytes via a type III secretion system. 2. Once attached, they form attaching and effacing lesions on the microvilli, leading to microvillus destruction. 3. This mechanism allows EPEC to cause diarrhea by disrupting the normal absorptive function of the intestines. Summary: - B (They are invasive and produce enterotoxin) is incorrect because EPEC are non-invasive and do not produce enterotoxins. - C (Cause disease clinically similar to bacterial) is vague and does not provide specific information about EPEC pathogenesis. - D (None of the above) is incorrect as A is the correct explanation for EPEC pathogenicity.

Correct Answer: B

Rationale: The correct answer is B: RG2. RG2 is appropriate for agents associated with human disease that pose a moderate health risk. These agents may cause disease of varying severity, but generally do not result in serious health consequences. Other choices are incorrect because: A: RG1 is for agents not associated with disease in healthy adults, C: RG3 is for agents associated with serious or lethal human disease, and D: RG4 is for agents that pose a high risk of life-threatening disease and for which there are no treatments.

Correct Answer: A

Rationale: The correct answer is A: Neisseria meningitidis. This bacterium is a Gram-negative diplococcus commonly associated with meningitis. Neisseria meningitidis is a leading cause of bacterial meningitis in young adults and adolescents. It is known for causing fever, headache, and neck stiffness. The other choices, Streptococcus pneumoniae, Haemophilus influenzae, and Klebsiella pneumoniae, are not typically Gram-negative diplococci and are not commonly associated with the symptoms described. Streptococcus pneumoniae is a Gram-positive coccus, Haemophilus influenzae is a Gram-negative coccobacillus, and Klebsiella pneumoniae is a Gram-negative rod. Therefore, Neisseria meningitidis is the most likely causative agent in this scenario.

Correct Answer: B

Rationale: The correct answer is B because a viral capsid is a protein coat that surrounds and protects the viral nucleic acid (DNA or RNA). It helps in the attachment of the virus to host cells and protects the genetic material during transmission. Choice A is incorrect because a viral capsid is composed of proteins, not an envelope. Choice C is incorrect because not all viruses have an outer membrane. Choice D is incorrect because a viral capsid is primarily made up of proteins, not lipids.

Correct Answer: D

Rationale: The correct answer is D: Genus Aspergillus. Aflatoxins are toxic compounds produced by certain species of fungi, particularly Aspergillus species. Aspergillus flavus and Aspergillus parasiticus are well-known producers of aflatoxins. These fungi can contaminate crops such as peanuts, corn, and tree nuts, leading to potential health risks. Cryptococcus (A), Actinomyces (B), and Streptococcus (C) are not known to produce aflatoxins. It is important to be aware of the sources and implications of aflatoxin contamination to ensure food safety and public health.

Correct Answer: B

Rationale: The correct answer is B: Linnaeus. Linnaeus developed the binomial system of nomenclature in the 18th century. He introduced the use of a two-part naming system consisting of a genus name and a species name, known as binomial nomenclature. This system helps in organizing and categorizing living organisms systematically. Pasteur (choice A) is known for his contributions to microbiology, not nomenclature. Martini (choice C) and Jenner (choice D) are not associated with the development of the binomial system of nomenclature.

Correct Answer: D

Rationale: The correct answer is D because all three bacteria listed (Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa) can produce endotoxins that can lead to septic shock. Endotoxins are lipopolysaccharides found in the outer membrane of Gram-negative bacteria like E. coli and P. aeruginosa. Staphylococcus aureus, a Gram-positive bacterium, can also produce endotoxins called superantigens that can trigger a massive immune response leading to septic shock. Therefore, all of the above choices are correct. Choices A, B, and C alone are incorrect because they do not account for the fact that all three bacteria listed produce endotoxins that can cause septic shock.