Which of the following is a host-dependent bacterium?

Correct Answer: C

Rationale: The correct answer is C: Rickettsia. Rickettsia is a host-dependent bacterium because it requires living cells of a host organism to survive and replicate. This bacterium is an obligate intracellular pathogen, meaning it cannot grow outside of a host cell. A: Mycoplasma and B: Ureaplasma are not host-dependent as they can grow and reproduce in cell-free environments due to possessing their own unique cell wall structures. D: Micrococcus is a free-living bacterium that can survive and reproduce independently without relying on a host organism.

Correct Answer: B

Rationale: The correct answer is B: antitoxin. Antitoxin is the specific antidote to botulinum toxin as it helps neutralize the toxin in the body. Sodium bicarbonate (A) is used to treat acidosis, pralidoxime (C) is used for organophosphate poisoning, and naloxone (D) is used for opioid overdose. Antitoxin directly targets and counteracts the botulinum toxin, making it the appropriate antidote in cases of botulism.

Correct Answer: A

Rationale: The correct answer is A: incidence. Incidence refers to the number of new cases of a disease occurring within a specific time period in relation to the healthy population. It helps in understanding the risk of developing the disease. Prevalence (B) is the total number of existing cases at a given point in time. Virulence (C) is the severity of the disease once contracted. Mortality (D) refers to the number of deaths due to the disease, not the new cases. Therefore, the correct term for new cases is incidence.

Correct Answer: B

Rationale: The correct answer is B, Helicobacter pylori. This bacterium is the main cause of peptic ulcers by damaging the protective lining of the stomach and duodenum. It secretes enzymes that weaken the mucosal barrier, allowing stomach acid to cause damage. Salmonella typhi (A) causes typhoid fever, not peptic ulcers. Shigella boydii (C) causes dysentery, not peptic ulcers. Salmonella enterica (D) causes food poisoning, not peptic ulcers. Therefore, the correct answer is B based on its direct association with peptic ulcer formation.

Correct Answer: B

Rationale: The correct answer is B: Rickettsia rickettsii. Rocky Mountain spotted fever is caused by Rickettsia rickettsii, a bacterium transmitted by ticks. This pathogen primarily affects the endothelial cells of blood vessels, leading to vasculitis. Borrelia burgdorferi (choice A) causes Lyme disease, not Rocky Mountain spotted fever. Rickettsia prowazekii (choice C) causes epidemic typhus, and Ehrlichia chaffeensis (choice D) causes human monocytic ehrlichiosis, both of which are distinct from Rocky Mountain spotted fever. Therefore, the correct choice is B based on the specific etiology of the disease.