Correct Answer: B

Rationale: The correct answer is B: Capsule. Capsules are a protective layer outside the bacterial cell wall that helps bacteria evade phagocytosis by immune cells. The capsule inhibits phagocytic cells from engulfing the bacteria, making it difficult for them to be recognized and destroyed. A: Flagella - Flagella are not involved in protecting bacteria from phagocytosis. They are responsible for bacterial movement. C: Endospore - Endospores are dormant, resistant structures formed by some bacteria in response to harsh conditions. They do not directly protect against phagocytosis. D: Pili - Pili are hair-like structures used for adhesion and conjugation, not for protection against phagocytosis.

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: D

Rationale: The correct answer is D because viruses are sensitive to interferon, a natural defense mechanism of the body against viral infections. Interferon helps to inhibit viral replication. A: Viruses are cell-free infectious particles, not living systems. B: Viruses are obligate intracellular parasites as they require host cells to replicate. C: Viruses can have either DNA or RNA as genetic material, which is a characteristic feature.

Correct Answer: D

Rationale: The correct answer is D: protozoan. Malaria is caused by Plasmodium parasites transmitted through the bite of infected female Anopheles mosquitoes. These parasites belong to the protozoa kingdom, not viruses (choice A), bacteria (choice B), or helminths (choice C). Protozoa have complex life cycles involving both mosquito and human hosts, leading to the development of malaria in humans. Other choices are incorrect as they do not align with the known etiology and characteristics of malaria.

Correct Answer: A

Rationale: The correct answer is A: Clostridium botulinum. Clostridium botulinum produces the botulinum toxin, which causes botulism. This bacteria thrives in anaerobic environments and can contaminate improperly canned foods. Clostridium tetani causes tetanus, not botulism. Staphylococcus aureus produces enterotoxins causing food poisoning, not botulism. Escherichia coli can cause food poisoning through various toxins, but not botulism. Therefore, the correct choice is Clostridium botulinum as it specifically produces the botulinum toxin responsible for botulism.