The typhoid fever is produced mainly by:

Correct Answer: D

Rationale: The correct answer is D: Salmonella typhi. Typhoid fever is caused by the bacterium Salmonella typhi. This bacterium is responsible for causing the systemic infection that leads to symptoms such as high fever, abdominal pain, and diarrhea. Yersinia pestis (choice A) causes plague, Shigella dysenteriae (choice B) causes dysentery, and Escherichia coli (choice C) can cause various gastrointestinal infections, but not typhoid fever. Salmonella typhi specifically targets the intestines and is transmitted through contaminated food or water. Therefore, choice D is the correct answer for the cause of typhoid fever.

Correct Answer: C

Rationale: Legionella pneumophila is a Gram-negative coccobacillus. It is not a coccus (round shape), spiral bacterium, or Gram-positive coccobacillus. The correct answer is C because Legionella pneumophila has a characteristic coccobacillary shape and stains Gram-negative due to its cell wall composition. The other choices are incorrect because they do not accurately describe the morphology or staining characteristics of Legionella pneumophila.

Correct Answer: D

Rationale: Rationale: 1. Rickettsiae are obligate intracellular bacteria, hence they do not form spores, capsules, or flagella. 2. They have a slim glycocalyx layer, aiding in attachment to host cells. 3. Bipolar fimbriae are structures that help in adhesion to host cells. Therefore, all statements (A, B, and C) are true for Rickettsiae, making option D the correct answer. Other choices are incorrect as they do not accurately describe the characteristics of Rickettsiae.

Correct Answer: C

Rationale: The correct answer is C because Leptospira cannot be cultivated on blood and chocolate agar. The bacterium requires specialized media such as Ellinghausen-McCullough-Johnson-Harris (EMJH) medium for growth. Blood and chocolate agar do not provide the necessary nutrients for the bacterium's growth. Leptospira is a fastidious organism that has unique culture requirements. Therefore, the statement that Leptospira can be cultivated on blood and chocolate agar is incorrect. The other choices are correct: A - Leptospirosis is an acute infectious disease with a typical geographic distribution, B - Reservoirs of leptospira are different wild and domestic animals, mainly rodents, and D - Doxycycline and penicillins are drugs of choice for the treatment of leptospirosis.

Correct Answer: B

Rationale: The correct answer is B: Reverse transcriptase and protease. Reverse transcriptase converts viral RNA into DNA, a crucial step in viral replication. Protease is responsible for cleaving viral polyproteins into functional proteins. Targeting both enzymes disrupts viral replication. Choice A is incorrect as integrase is not a main target in current therapy. Choice C is incorrect because protease is targeted along with reverse transcriptase, not integrase. Choice D is incorrect as targeting viral glycoproteins is not a main strategy in anti-HIV therapy.