Correct Answer: A

Rationale: The correct answer is A: Leptospira. The key features pointing to Leptospira are the presence of convoluted bacteria resembling C and S letters, which is characteristic of Leptospira species. Leptospirosis presents with fever, jaundice, haemorrhagic rash, and acute renal insufficiency. The other choices (B: Treponema, C: Spirilla, D: Borrelia) do not match the histological description or the clinical presentation of the case. Treponema is associated with syphilis, Spirilla with genus Spirochaeta, and Borrelia with Lyme disease, none of which match the given scenario.

Correct Answer: C

Rationale: Rationale: 1. Vaccinia is a laboratory product used in smallpox vaccination. 2. Variola is the virus that causes smallpox, not a laboratory product. 3. Monkeypox is a naturally occurring virus, not a laboratory product. 4. Cowpox can be found in nature and is used in vaccine production, but it is not a laboratory product like Vaccinia.

Correct Answer: C

Rationale: The correct answer is C: Louis Pasteur. He conducted experiments that definitively disproved the theory of spontaneous generation through his famous swan-neck flask experiment. By showing that microorganisms only appeared in the broth when exposed to air, Pasteur demonstrated that they did not arise spontaneously. John Needham's experiments were inconclusive, Joseph Lister is known for his contributions to antiseptic surgery, and Robert Koch is credited with developing Koch's postulates for identifying the causative agents of diseases, but neither of them directly contributed to ending the controversy about spontaneous generation.

Correct Answer: C

Rationale: The correct answer is C: S.aureus. Stomatitis is commonly caused by S.aureus due to its ability to form clusters of oval polymorphous Gram-positive microorganisms. S.aureus is known to colonize the oral mucous membrane and can cause infections, especially after prolonged antibiotic use. Explanation for other choices: A: С.albicans - Candida albicans is a fungus commonly associated with oral thrush, not typically causing stomatitis. B: C.perfringens - Clostridium perfringens is a bacterium associated with food poisoning and gas gangrene, not commonly causing stomatitis. D: S.pyogenes - Streptococcus pyogenes is known to cause strep throat and skin infections, but not typically associated with stomatitis.

Correct Answer: D

Rationale: The correct answer is D: Clindamycin. Clindamycin is a lincosamide antibiotic that is safe to use in patients with a history of anaphylaxis to beta-lactam antibiotics like cefaclor. Clindamycin is effective against streptococcus pyogenes and is a suitable alternative in this case. Amoxicillin (choice A) is a beta-lactam antibiotic similar to cefaclor and may cause a cross-reactive allergic reaction. Meropenem (choice B) is a carbapenem antibiotic and not typically used for acute pharyngitis. Cephalexin (choice C) is a cephalosporin antibiotic and may also cause cross-reactivity in patients with a history of anaphylaxis to cefaclor.