Correct Answer: A

Rationale: The correct answer is A: BCG. BCG vaccine is commonly used for tuberculosis prevention because it provides immunity against Mycobacterium tuberculosis. Mantoux is a test, not a vaccine. DTaP vaccine is for diphtheria, tetanus, and pertussis, not tuberculosis. Anatoxin is a type of toxoid vaccine, not used for tuberculosis prevention. BCG vaccine has been shown to be effective in preventing severe forms of tuberculosis in infants, making it the appropriate choice in this scenario.

Correct Answer: A

Rationale: Step 1: Gram-positive cocci in chains suggest Streptococcus species. Step 2: Catalase-negative eliminates Staphylococcus aureus. Step 3: Beta-hemolytic on blood agar matches Streptococcus pyogenes. Step 4: Streptococcus pyogenes is a common cause of sore throat (pharyngitis). Summary: A: Streptococcus pyogenes - Correct due to Gram-positive cocci in chains, catalase-negative, beta-hemolytic, and common cause of sore throat. B: Staphylococcus aureus - Incorrect due to catalase-positive. C: Streptococcus pneumoniae - Incorrect due to alpha-hemolytic. D: Enterococcus faecalis - Incorrect due to not being beta-hemolytic.

Correct Answer: D

Rationale: The correct answer is D because bacteremia can be caused by a variety of bacteria including S.epidermidis, E.coli, S.aureus, P.aeruginosa, K.pneumoniae, and resident skin diphtheroids. Bacteremia refers to the presence of bacteria in the bloodstream, which can occur due to infections from various sources. Choices A and B include common bacteria known to cause bacteremia, while choice C refers to resident skin diphtheroids that can also enter the bloodstream under certain conditions. Therefore, the correct answer is D as it encompasses all the possible etiological causes of bacteremia.

Correct Answer: B

Rationale: The correct answer is B: Alpha-hemolytic streptococci. Streptococcus pneumoniae are classified as alpha-hemolytic streptococci based on their ability to partially break down red blood cells, causing a greenish discoloration on blood agar plates. This distinguishes them from beta-hemolytic streptococci, which completely break down red blood cells. Streptococcus pneumoniae are not resistant to bile (choice A), as they are bile soluble. They are not stained by the Ziehl-Neelsen staining method (choice C), which is used for acid-fast bacteria such as Mycobacterium tuberculosis. They are sensitive to optochin (choice D), which is a key differentiating factor in identifying them from other alpha-hemolytic streptococci.

Correct Answer: C

Rationale: The correct answer is C because infectious (non-sterile) immunity is achieved through the persistence of the infectious agent in the body, allowing the immune system to continuously recognize and combat the pathogen. This type of immunity provides long-lasting protection against future infections by the same agent. Choice A (cross-reactivity of antibodies) is incorrect as it refers to the ability of antibodies to recognize similar antigens, not necessarily related to infectious immunity. Choice B (toxemia and formation of antitoxic antibodies) is incorrect as it specifically relates to toxins produced by pathogens and the corresponding antibody response, not the persistence of the infectious agent itself. Choice D (neutralization of infectious agent) is incorrect as it describes the process of antibodies binding to and inactivating pathogens, but it does not necessarily lead to long-term immunity through the persistence of the infectious agent.