Correct Answer: B

Rationale: The correct answer is B: Phylum. After class in the classification system comes phylum. The classification system starts with the broadest category, which is the kingdom, followed by phylum, class, order, family, genus, and species. Phylum is a higher level of classification than class, so it comes next in the hierarchy. Domain is a higher level than kingdom, and order is a lower level of classification than class. Therefore, the correct sequence is kingdom, phylum, class, order.

Correct Answer: A

Rationale: The correct answer is A: Peroxidase and catalase. Peroxidase and catalase are enzymes that break down hydrogen peroxide into water and oxygen. Peroxidase helps in breaking down low levels of hydrogen peroxide, while catalase is crucial for breaking down high levels of hydrogen peroxide efficiently. This process is important for organisms to prevent damage from reactive oxygen species. Choices B, C, and D are incorrect: B: Oxygenase and hydroxylase are enzymes involved in different biochemical reactions and do not break down hydrogen peroxide. C: Cytochrome oxidase and cytochrome B5 are involved in electron transport chain reactions and not in breaking down hydrogen peroxide. D: Oxygenase and catalase are not correct as oxygenase is not involved in hydrogen peroxide breakdown, only catalase is.

Correct Answer: C

Rationale: The correct answer is C because diphtheria exotoxin is absorbed into the underlying healthy tissues and spreads throughout the body. This is due to the toxin's ability to enter the bloodstream and target various organs, causing systemic effects. Choice A is incorrect as the toxin does not stay localized at the site of infection. Choice B is incorrect as diphtheria exotoxin does not directly affect neuro-muscular transmission. Choice D is also incorrect as the toxin does spread throughout the body.

Correct Answer: D

Rationale: The correct answer is D: Agglutination test type Gruber. This is because the Gruber agglutination test is not typically used for the serological identification of Yersinia pestis. The other options (A, B, C) are commonly used methods for serological identification of Y. pestis: A) Precipitation reaction involves the formation of a visible precipitate when Y. pestis antigen reacts with specific antibodies. B) Indirect hemagglutination test detects antibodies against Y. pestis by measuring the agglutination of red blood cells. C) Immunofluorescence test uses fluorescently labeled antibodies to detect specific antigens of Y. pestis. In summary, the Gruber agglutination test is not a standard method for serological identification of Yersinia pestis, unlike the other options provided.

Correct Answer: A

Rationale: The correct answer is A: Entamoeba gingivalis. This organism matches the description provided in the question - unicellular with pseudopodia and ingested white blood cells, commonly found in dental plaque. Entamoeba gingivalis is known to inhabit the oral cavity and is associated with periodontal disease. Trichomonas vaginalis, Giardia lamblia, and Balantidium coli do not match the characteristics mentioned in the question and are not typically found in the oral cavity.