Erysipelas is a bacterial infection characterised by well-defined areas of bright-red, inflamed and rough or leathery skin. Which one is the causative agent:

Correct Answer: C

Rationale: Step-by-step rationale for Choice C (Streptococcus pyogenes) being the correct answer: 1. Erysipelas is a skin infection characterized by well-defined red, inflamed skin areas. 2. Streptococcus pyogenes is a common causative agent of erysipelas. 3. This bacterium produces enzymes that break down skin proteins, leading to the characteristic symptoms of erysipelas. 4. Streptococcus pyogenes is known for causing skin infections, including erysipelas. Summary of why the other choices are incorrect: A. Pseudomonas aeruginosa: Typically causes opportunistic infections, but not commonly associated with erysipelas. B. Staphylococcus aureus: Can cause skin infections, but not typically associated with erysipelas. D. Candida albicans: Fungal infection, not a bacterium, and not known to cause erysipelas.

Correct Answer: B

Rationale: The correct answer is B: Chlamydia trachomatis. Elementary bodies (EBs) and reticular bodies (RBs) are two distinct forms of Chlamydia trachomatis, a bacterium causing various infections. EBs are the infectious form, while RBs are the replicative form within host cells. Mycoplasma genitalium (choice A) lacks a cell wall and does not have these specific morphological forms. Rickettsia prowazekii (choice C) is an obligate intracellular bacterium but does not exhibit the elementary and reticular body forms characteristic of Chlamydia trachomatis. Therefore, the correct answer is B, as only Chlamydia trachomatis exhibits the elementary and reticular bodies.

Correct Answer: A

Rationale: Step 1: Poxviruses are indeed DNA viruses, confirmed by scientific research. Step 2: Poxviruses replicate entirely in the cytoplasm, unlike most DNA viruses. Step 3: Cytoplasmic replication is a unique characteristic of poxviruses. Step 4: The replication process of poxviruses in the cytoplasm is well-documented. Step 5: Therefore, option A (TRUE) is correct, as poxviruses replicate in the cytoplasm exclusively.

Correct Answer: A

Rationale: The correct answer is A: Pseudomonas aeruginosa. P. aeruginosa is known for producing a blue-green pigment called pyocyanin, which gives a characteristic color to the colonies. The fruity odor is due to the production of a compound called 2-aminoacetophenone by P. aeruginosa. These features are classic for P. aeruginosa and help differentiate it from other Gram-negative rods. Proteus mirabilis (B) typically does not produce blue-green pigment or fruity odor. Klebsiella pneumoniae (C) and Escherichia coli (D) also do not exhibit these specific characteristics associated with P. aeruginosa.

Correct Answer: C

Rationale: The correct answer is C: Plague. Yersinia pestis is the bacterium responsible for causing the bubonic, septicemic, and pneumonic plague. The bacterium is transmitted to humans through infected flea bites or handling infected animals. Tonsillitis (A) is commonly caused by streptococcal bacteria, leprosy (B) is caused by Mycobacterium leprae, and rat fever (D) is another term for leptospirosis, caused by Leptospira bacteria.

Correct Answer: A

Rationale: The correct answer is A: C. perfringens. This is because Clostridium perfringens is a gram-positive, anaerobic, spore-forming bacterium commonly associated with food poisoning. It produces toxins that cause gastrointestinal symptoms. Proteus vulgaris (B) and P. mirabilis (C) are gram-negative bacteria not typically associated with food poisoning. Vibrio parahemolyticus (D) is a gram-negative bacterium causing seafood-related gastroenteritis, not spore-forming or anaerobic.

Correct Answer: D

Rationale: The correct answer is D: Innate immunity (natural resistance). Innate immunity refers to the non-specific defense mechanisms that act against a variety of infectious agents. This includes physical barriers like skin and mucous membranes, as well as cellular components like phagocytes and natural killer cells. Adaptive immunity (A) is specific and involves the production of antibodies and immune memory (B), which are not part of the initial non-specific response. Antibodies (C) are part of the adaptive immune response and are produced in response to specific pathogens, not part of the non-specific innate defense factors.

Correct Answer: C

Rationale: The correct answer is C (coli index of 250 and coli titer of 4). The coli index is calculated by multiplying the number of colonies by the reciprocal of the volume filtered, which gives 2 colonies x (1/0.5 L) = 4. The coli titer is obtained by dividing the volume filtered by the number of colonies, which gives 0.5 L / 2 colonies = 0.25 L or 250 ml. This indicates the concentration of coliform bacteria in the water sample. Choice A (4 and 250) is incorrect because it has the correct coli index but the values are reversed for coli index and coli titer. Choice B (2 and 500) is incorrect as it provides the correct coli titer value but the values are reversed for coli index and coli titer. Choice D (500 and 2) is incorrect because it reverses the values for both coli index and coli titer.

Correct Answer: A

Rationale: The correct answer is A because adaptive immunity is not mainly observed in the skin and mucous membranes. Adaptive immunity is a systemic response that involves specific immune cells and antibodies targeting specific pathogens. It develops immune memory (B), meaning it can recognize and respond faster upon re-exposure to the same pathogen. Adaptive immunity also has specificity (C), as it can target specific antigens. Lastly, adaptive immunity is not evolutionarily newer in comparison to innate immunity (D), as both systems have co-evolved to provide comprehensive protection against pathogens.