A fecal smear from a patient with severe diarrhea revealed motile Gram-negative rods with a comma shape. What is the likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Vibrio cholerae. This bacterium is the likely causative agent because it is known to cause severe diarrhea, and it is characterized by its motile, comma-shaped Gram-negative rods. Vibrio cholerae produces cholera toxin, leading to profuse watery diarrhea. Shigella dysenteriae causes dysentery with bloody diarrhea, Salmonella typhi causes typhoid fever, and Escherichia coli can cause various types of diarrhea but typically does not have a comma shape.

Correct Answer: B

Rationale: Binary fission is a form of asexual reproduction where a single organism divides into two equal parts. Bacteria reproduce through binary fission, splitting into two identical daughter cells. Viruses are not considered living organisms and reproduce by infecting host cells. Fungi reproduce through spores or sexual reproduction. Helminths are parasitic worms that reproduce sexually. Therefore, the correct answer is B, bacteria, as they specifically utilize binary fission for reproduction.

Correct Answer: B

Rationale: The correct answer is B: Staphylococcus aureus. This bacterium is known as a common cause of hospital-acquired infections due to its ability to colonize the skin and mucous membranes of healthcare workers and patients. Staphylococcus aureus can cause various infections, including surgical site infections and bloodstream infections, in healthcare settings. Rationale: 1. Clostridium botulinum is known for causing foodborne botulism, not hospital-acquired infections. 2. Treponema pallidum causes syphilis, which is primarily transmitted through sexual contact, not hospital settings. 3. Mycobacterium leprae causes leprosy, which is not commonly associated with hospital-acquired infections. In summary, Staphylococcus aureus is the correct answer as it is a common cause of hospital-acquired infections due to its prevalence in healthcare environments, unlike the other choices which are not typically associated with hospital-acquired infections.

Correct Answer: A

Rationale: The correct answer is A: HIV. HIV is an example of obligate parasitism because it requires a host cell to replicate and survive. It cannot live independently outside of a host organism. In contrast, choices B, C, and D are not obligate parasites. E. Coli and S. Epidermidis are examples of commensal bacteria that can live in the host without causing harm. Pseudomonas aeruginosa can be both an opportunistic pathogen and a free-living organism, not strictly dependent on a host for survival.

Correct Answer: A

Rationale: Rationale: 1. Spore-forming, Gram-positive rods: Characteristics shared by Clostridium and Bacillus. 2. Motile and gas production in nutrient broth: Clostridium perfringens is motile and produces gas, distinguishing it from Bacillus species. 3. Clostridium perfringens causes gas gangrene, correlating with gas production. 4. Bacillus anthracis causes anthrax, not related to wound infections. 5. Clostridium tetani causes tetanus, not associated with gas production. 6. Bacillus cereus causes food poisoning, not commonly associated with wound infections. Summary: - A is correct (Clostridium perfringens) due to motility, gas production, and wound infection correlation. - B (Bacillus anthracis) causes anthrax, not wound infections. - C (Clostridium tetani) causes tetanus, not gas production. - D (B

Correct Answer: A

Rationale: Rationale: - Yersinia enterocolitica is a common cause of diarrhea due to contaminated food or water. - It typically presents with symptoms such as abdominal pain, fever, and diarrhea. - The other choices are unlikely causes of prolonged diarrhea, as Bordetella pertussis causes whooping cough, Neisseria meningitidis causes meningitis, and Corynebacterium diphtheria causes diphtheria.

Correct Answer: D

Rationale: The correct answer is D: Blood. Normal flora refers to the microorganisms that naturally inhabit specific areas of the body. Blood is considered a sterile environment in healthy individuals, meaning it does not harbor any microorganisms under normal circumstances. In contrast, the intestine, skin, and vagina all have normal flora as they provide suitable environments for various microbial species to thrive. The presence of normal flora in these sites helps maintain a balance and protect against pathogenic invaders. Blood, being a vital and sterile fluid, is not meant to have any resident microbial populations.

Correct Answer: B

Rationale: The correct answer is B: Agglutination reaction. This test is commonly used to detect specific antibodies or antigens. In this case, it can be used to detect plague antigens in the rat tissues or antibodies in the villagers. This test is rapid, sensitive, and can help establish the presence of the causative agent quickly. A: Ring precipitation reaction is not typically used for diagnosing plague and is more commonly used in other types of infections. C: Passive agglutination reaction is not specific for plague and is more commonly used for detecting specific antibodies in diseases like syphilis. D: Neutralization reaction is not typically used for diagnosing plague and is more commonly used to determine the presence of neutralizing antibodies in viral infections.

Correct Answer: A

Rationale: Step-by-step rationale: 1. Mumps is a contagious disease transmitted through air droplets and direct contact. 2. The correct answer is A because mumps can infect both humans and animals. 3. Choice B is incorrect as mumps is vaccine-preventable. 4. Choice C is correct as mumps is highly contagious. 5. Choice D is accurate as mumps is transmitted via air droplets and direct contact. Summary: The correct answer is A because mumps can infect both humans and animals, making it untrue that only humans can be infected. Choices B, C, and D are incorrect as mumps is vaccine-preventable, highly contagious, and transmitted through air droplets and direct contact.