A patient with fever and sore throat had a throat culture revealing Gram-positive cocci in chains. The bacteria were catalase-negative and beta-hemolytic. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pyogenes. 1. Gram-positive cocci in chains: characteristic of Streptococcus species. 2. Catalase-negative: Streptococcus species are catalase-negative. 3. Beta-hemolytic: S. pyogenes is beta-hemolytic on blood agar. Therefore, S. pyogenes fits all these criteria. B: Streptococcus pneumoniae is catalase-negative but alpha-hemolytic, not beta-hemolytic. C: Enterococcus faecalis is catalase-negative, but not typically beta-hemolytic or in chains. D: Staphylococcus aureus is catalase-positive and not in chains.

Correct Answer: A

Rationale: Rationale: 1. Iodine solution contains iodine, a halogen with antiseptic properties. 2. Iodine is effective against fungi, making it suitable for treating dermatomycosis. 3. Iodine solution is commonly used in dermatology for its fungicidal properties. 4. Formalin, methylene blue, and brilliant green do not contain halogens or have fungicidal properties. Summary: - Formalin is a disinfectant, not an antiseptic for dermatomycosis. - Methylene blue is an antifungal dye, not used as an antiseptic. - Brilliant green is an antiseptic, but does not contain halogens or have fungicidal properties.

Correct Answer: B

Rationale: The correct answer is B: Shigella. On Endo agar, Shigella appears as colorless colonies due to its inability to ferment lactose. Shigella is a Gram-negative rod and is a common cause of diarrhea. Salmonella (A) and Escherichia coli (C) would typically produce pink colonies on Endo agar as they can ferment lactose. Proteus (D) is not commonly associated with diarrhea and would not be expected to grow on Endo agar.

Correct Answer: B

Rationale: Rationale: The correct answer is B because virulence factors of bacteria are characteristics that enable them to cause disease. Colonization allows bacteria to establish and multiply in the host. Invasion refers to the ability to penetrate host tissues. Toxins are substances produced by bacteria that harm the host. Adhesion is crucial for bacteria to attach to host cells. A: Size and mutation are not typical virulence factors. Resistance is not directly related to causing disease. C: While toxins and evasion of body defenses are correct, motility and size are not universally considered virulence factors. D: Chemotaxis and fermentation are not primary virulence factors, unlike colonization and toxins.

Correct Answer: B

Rationale: The correct answer is B: protein filaments made of flagellin. Bacterial flagella are composed of flagellin proteins arranged in helical filaments. This structure allows for movement and propulsion of bacteria. A: 9+2 microtubule arrangement is incorrect because this structure is typically found in eukaryotic flagella, not bacterial flagella. C: Lipid bilayer is incorrect as it is a structural component of cell membranes, not bacterial flagella. D: Peptidoglycan layer is incorrect as it is a component of bacterial cell walls, not flagella.

Correct Answer: C

Rationale: The correct answer is C: cytoplasmic membrane. The cytoplasmic membrane is the fluidlike portion of a cell that separates the cell's contents from the external environment. It regulates the passage of substances in and out of the cell, maintains cell shape, and provides protection. Rationale: 1. Cytosol (choice A) is the liquid component of the cytoplasm where organelles are suspended, not the fluidlike portion of the cell. 2. Nucleoplasm (choice B) refers to the liquid component of the nucleus, not the entire cell. 3. Inclusion (choice D) refers to non-living substances within the cell that are not considered the fluidlike portion. Summary: The cytoplasmic membrane is the correct answer as it encompasses the entire cell and is responsible for maintaining cell integrity and regulating transport. Choices A, B, and D are incorrect as they do not represent the fluidlike portion of the cell.

Correct Answer: D

Rationale: The correct answer is D: All are correct. Culture of samples from the respiratory tract can identify specific pathogens, Immunofluorescence can detect viral antigens, and PCR can amplify and detect specific DNA/RNA sequences. Using all methods together provides a comprehensive approach for diagnosing respiratory tract infections. Culture alone may miss fastidious organisms, Immunofluorescence has limitations in sensitivity, and PCR requires specific primers. Therefore, combining these methods increases the likelihood of accurate diagnosis by covering a range of pathogens and improving sensitivity and specificity.

Correct Answer: B

Rationale: The capsule of Bacillus anthracis is made of polypeptide. This is because the capsule is composed of a protein-based material that surrounds the bacterium, providing protection and aiding in its virulence. Polysaccharides, lipids, and the absence of capsules are not characteristic of Bacillus anthracis' capsule structure, making choices A, C, and D incorrect, respectively.

Correct Answer: B

Rationale: The correct answer is B: Precipitins. When a granular sediment forms at the bottom of the test-tubes, it indicates the formation of antigen-antibody complexes that are insoluble and precipitate out of the solution. Precipitins are antibodies that specifically react with antigens to form insoluble complexes. Agglutinins cause clumping of cells, opsonins enhance phagocytosis, and bacteriolysins lyse bacteria. These processes do not result in the formation of a granular sediment, making them incorrect choices in this context.