Microbial life on teeth was first observed by:

Correct Answer: C

Rationale: The correct answer is C: Van Leeuwenhoek. Van Leeuwenhoek was a Dutch scientist known for his pioneering work in microscopy. He was the first to observe microbial life, including bacteria on teeth, using his handmade microscopes in the 17th century. Jenner (choice A) is known for developing the smallpox vaccine, not for microbial observations. Semmelweis (choice B) discovered the importance of handwashing in preventing infections. Pasteur (choice D) is famous for his work on pasteurization and germ theory, but not specifically for observing microbial life on teeth. Van Leeuwenhoek's meticulous observations and groundbreaking discoveries make him the correct choice in this context.

Correct Answer: D

Rationale: The correct answer is D because bacterial vaginitis is often caused by an imbalance in the vaginal flora, which can be disrupted by frequent douching (eliminating beneficial bacteria), tight clothing (restricting airflow), and non-breathable underwear. Avoiding tight pants and using cotton underwear allow for better ventilation, promoting a healthy vaginal environment. Infrequent douching helps maintain the natural balance of bacteria in the vagina. Choices A and B are incorrect because they recommend practices that can disrupt the vaginal flora and increase the risk of bacterial vaginitis. Choice C is partially correct but lacks the emphasis on avoiding frequent douching, which is crucial in preventing bacterial vaginitis.

Correct Answer: D

Rationale: The correct answer is D: polyomavirus infections. Polyomavirus infections do not typically recur during pregnancy because once the virus infects the body, it remains latent without causing recurrent infections. In contrast, choices A, B, and C can recur during pregnancy due to their ability to reactivate from latency or reinfect the host. Epstein-Barr virus infections (choice A) can lead to infectious mononucleosis, herpes simplex virus infections (choice B) can cause recurrent genital or oral herpes outbreaks, and streptococcal infections (choice C) can result in recurrent throat or skin infections.

Correct Answer: A

Rationale: The correct answer is A: tRNA. Anticodons are sequences of nucleotides found on tRNA molecules that complement the codons on mRNA during translation. tRNA molecules carry specific amino acids to the ribosome based on the codon-anticodon pairing. Option B, rRNA, forms the structure of the ribosome and is not directly involved in anticodon recognition. Option C, mRNA, contains codons but not anticodons. Option D, ribosomes, are the site of protein synthesis but do not contain the anticodons themselves.

Correct Answer: B

Rationale: The correct answer is B: Hantavirus. Hantaviruses are not arthropod-borne, unlike the other choices. Hantaviruses are transmitted to humans primarily through contact with rodent urine, feces, or saliva. Phlebovirus (A), Nairovirus (C), and Uukuvirus (D) are all bunyaviruses that are transmitted by arthropod vectors such as mosquitoes or ticks. Therefore, Hantavirus is the correct answer as it is not transmitted by arthropods.