Correct Answer: D

Rationale: The correct answer is D because there is specific prophylaxis available for influenza, such as the flu vaccine. Choice A is incorrect as influenza viruses are not spherical, they are typically pleomorphic. Choice B is incorrect because hemagglutinin and neuraminidase are indeed antigenically variable, contributing to the need for annual flu vaccines. Choice C is incorrect as the susceptibility of the population to influenza viruses can vary depending on factors such as immunity and vaccination status.

Correct Answer: C

Rationale: Rationale: 1. Vibrio cholerae produces cholera toxin causing severe diarrhea. 2. Clostridium tetani causes tetanus, not diarrhea. 3. Clostridium botulinum causes botulism, not diarrhea. 4. Shigella dysenteriae causes dysentery, not typical diarrhea. Summary: A: Incorrect, causes tetanus. B: Incorrect, causes botulism. C: Correct, produces cholera toxin causing severe diarrhea. D: Incorrect, causes dysentery.

Correct Answer: B

Rationale: The correct answer is B: Pseudomonas aeruginosa. This bacterium is important for controlling air quality as it can degrade a wide range of organic compounds present in the air, helping to maintain a clean and healthy environment. Alpha and beta-hemolytic streptococci (Choice A) are not typically associated with air quality control. E. coli (Choice C) and Salmonella spp. (Choice D) are more commonly found in water and food sources, not air, making them incorrect choices for this question. Pseudomonas aeruginosa's ability to break down organic matter in the air makes it the most suitable option for maintaining sanitary air quality.

Correct Answer: B

Rationale: The correct answer is B: Vibrio cholerae. Cholera is caused by this specific bacterium due to its production of cholera toxin, leading to severe watery diarrhea. Shigella dysenteriae causes dysentery, Salmonella enterica causes salmonellosis, and Clostridium perfringens causes food poisoning, none of which present with the characteristic symptoms of cholera.

Correct Answer: A

Rationale: The correct answer is A: Helicobacter pylori. This bacterium is the most likely causative agent of gastric ulcers. Here's the rationale: 1. Helicobacter pylori is commonly associated with gastric ulcers. 2. It is a gram-negative bacterium that grows on chocolate agar and is oxide reductase-positive. 3. The slow growth on the fifth day is characteristic of H. pylori. 4. Campylobacter jejuni and Campylobacter fetus are not typically associated with gastric ulcers. 5. Mycoplasma hominis is a bacterium that lacks a cell wall and is not commonly found in gastric ulcers.