An adult patient with type 2 diabetes attends the emergency room. He reported that two days ago he was accidentally injured in the left leg, but he did not seek medical care. At the time of examination, an infected wound was found, surrounded by a large area of inflammation with characteristic tissue crepitation on palpation and with foul odor. Blood (for hemoculture) and wound secretions were taken for microbiological testing. The microscopic examination reveals large Gram-positive bacteria, with centrally located spores. These bacteria were unable to grow in vitro at presence of oxygen. The most likely cause of this disease is:

Correct Answer: B

Rationale: The correct answer is B: Clostridium perfringens. In this case, the patient's presentation of infected wound with tissue crepitation, foul odor, and Gram-positive bacteria with centrally located spores that cannot grow in the presence of oxygen is indicative of gas gangrene caused by Clostridium perfringens. This anaerobic bacterium produces various toxins that lead to tissue destruction and gas production in the affected area. Clostridium tetani (choice A) causes tetanus, characterized by muscle stiffness and spasms. Clostridium botulinum (choice C) causes botulism, leading to muscle paralysis. Clostridium difficile (choice D) is associated with antibiotic-associated diarrhea. These choices are incorrect as they do not match the clinical presentation described in the question.

Correct Answer: B

Rationale: The correct answer is B: Bordetella pertussis. This bacterium causes whooping cough by infecting the respiratory tract. It produces toxins that lead to severe coughing fits characteristic of the disease. Clostridium botulinum (A) causes botulism, Streptococcus pneumoniae (C) causes pneumonia, and Neisseria meningitidis (D) causes meningitis, not whooping cough. B. pertussis is the only bacterium specifically associated with whooping cough.

Correct Answer: B

Rationale: The correct answer is B: Pseudomonas aeruginosa. P. aeruginosa is known for producing pyocyanin, a green pigment. This bacterium is commonly associated with infections in immunocompromised individuals. Staphylococcus aureus (choice A) produces golden-yellow pigments, not green. Escherichia coli (choice C) and Clostridium difficile (choice D) do not produce pyocyanin. Therefore, the correct choice is B based on the characteristic pigment produced by Pseudomonas aeruginosa.

Correct Answer: B

Rationale: The correct answer is B: Clostridium perfringens. In this case, the patient's presentation of infected wound with tissue crepitation, foul odor, and Gram-positive bacteria with centrally located spores that cannot grow in the presence of oxygen is indicative of gas gangrene caused by Clostridium perfringens. This anaerobic bacterium produces various toxins that lead to tissue destruction and gas production in the affected area. Clostridium tetani (choice A) causes tetanus, characterized by muscle stiffness and spasms. Clostridium botulinum (choice C) causes botulism, leading to muscle paralysis. Clostridium difficile (choice D) is associated with antibiotic-associated diarrhea. These choices are incorrect as they do not match the clinical presentation described in the question.

Correct Answer: C

Rationale: Nigrosin is used in negative staining, where the background is stained instead of the cells themselves. This technique helps highlight the morphology and structure of cells that may be damaged by traditional staining methods. Nigrosin is negatively charged and repels the negatively charged bacterial cells, resulting in a clear halo around the cells. This process is particularly useful for observing capsules and determining cell size and shape. The other options are incorrect because simple staining uses a single dye to color the entire cell, gram staining involves a series of dyes to differentiate between gram-positive and gram-negative bacteria, and acid-fast staining is used to detect acid-fast organisms like Mycobacterium.

Correct Answer: B

Rationale: The correct answer is B: plasma coagulase. This enzyme is essential for the pathogenicity of Staphylococcus aureus as it can convert fibrinogen to fibrin, forming a protective barrier around the bacteria. This helps the bacteria evade the host's immune system. Beta lactamase (A) is not directly related to pathogenicity but rather resistance to beta-lactam antibiotics. Catalase (C) is produced by many bacteria, including Staphylococcus aureus, but it is not a specific indicator of pathogenicity. Choice D is incorrect as plasma coagulase is indeed an important criterion for the pathogenicity of Staphylococcus aureus.

Correct Answer: C

Rationale: The correct answer is C because Endo, Gassner, and Levin are all examples of differential media used in microbiology. These media contain specific components that allow for the differentiation of different types of microorganisms based on their metabolic characteristics or biochemical reactions. Lowenstein-Jensen and Petraniani (choice A) are selective media for mycobacteria, not differential. Apoholate-citrate agar and Brilliant-green phenol-red Lactose agar (choice B) are selective media for Salmonella and E. coli, respectively, not differential. Therefore, the correct choice is C as it includes examples of differential media commonly used for microbial identification.

Correct Answer: A

Rationale: The correct answer is A (25-37 °C) because mesophilic bacteria thrive best at moderate temperatures typical of most environments, making 25-37 °C their optimum range. This range allows for optimal enzyme activity and growth. Choice B (55-65 °C) is incorrect as it is more suitable for thermophilic bacteria. Choice C (15-18 °C) is too low for mesophiles to thrive efficiently. Choice D (38-45 °C) is too high and is more suitable for thermophiles. Ultimately, the correct answer aligns with the typical temperature range where mesophilic bacteria are most active and abundant.

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.