Correct Answer: B

Rationale: The correct answer is B: 3.5 to 4.0 billion years. Prokaryotic fossils date back to around 3.5 to 4.0 billion years ago, based on scientific evidence. This timeframe aligns with the earliest known evidence of life on Earth. Choices A, C, and D have incorrect timelines that do not correspond to the established age range of prokaryotic fossils. Choice A is too broad and includes a range that extends beyond the known fossil record. Choices C and D have much younger timelines, which do not match the historical evidence of prokaryotic life dating back billions of years.

Correct Answer: A

Rationale: The correct answer is A: Bordetella pertussis. Glossy smooth colonies growing on casein-charcoal agar resembling mercury drops indicate Bordetella pertussis, the causative agent of whooping cough. The presence of short Gram-negative bacteria further supports this identification. Option B, Haemophilus influenzae, typically grows as small, round, translucent colonies on chocolate agar. Option C, Corynebacterium diphtheriae, forms gray colonies on tellurite agar. Option D, Klebsiella pneumoniae, appears as mucoid colonies on MacConkey agar.

Correct Answer: A

Rationale: Rationale for Correct Answer A (Lamblia): 1. Pyriform protozoa with twin nuclei, four pairs of flagella, and a suctorial disc match the characteristics of Giardia lamblia. 2. The presence of two supporting filaments between the nuclei is a unique feature of Giardia lamblia. 3. Giardia lamblia is known to infect the duodenum and cause symptoms such as diarrhea and abdominal pain. Summary of Incorrect Choices: B: Toxoplasma - Toxoplasma gondii is not characterized by four pairs of flagella or supporting filaments in the nuclei. C: Leishmania - Leishmania species do not typically have the described morphology or location in the duodenum. D: Intestinal trichomonad - Intestinal trichomonads do not exhibit the specific features mentioned in the question, such as twin nuclei and supporting filaments.

Correct Answer: C

Rationale: Step-by-step rationale for why choice C (Nystatin) is the correct answer: 1. Nystatin is an antifungal medication specifically effective against Candida infections. 2. Candida is a type of fungus, not a bacteria targeted by penicillins (choice A) or cephalosporins (choice B). 3. Nystatin works by disrupting the fungal cell membrane, leading to its death. 4. Choice D (None of the above) is incorrect as Nystatin is a suitable treatment option for Candida infections.

Correct Answer: C

Rationale: Step-by-step rationale: 1. Shigella is a nonmotile bacterium. 2. This lack of motility is a key characteristic used to differentiate it from other enteric bacteria. 3. Nonmotile nature of Shigella helps it to cause infections by invading the intestinal epithelial cells. 4. Therefore, choice C is correct as Shigella is indeed nonmotile. Summary: A: Incorrect - Shigella does not produce hydrogen sulfide. B: Incorrect - Shigella does not ferment lactose. D: Incorrect - Shigella is not mannitol positive.