Microscopic examination of cerebrospinal fluid revealed Gram-negative diplococci. The patient presented with fever, headache, and neck stiffness. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Neisseria meningitidis. This bacterium is a Gram-negative diplococcus commonly associated with meningitis. The patient's symptoms of fever, headache, and neck stiffness are classic signs of meningococcal meningitis. Neisseria meningitidis is known to cause outbreaks in crowded settings. Streptococcus pneumoniae (Choice B) is a common cause of bacterial meningitis but typically appears as Gram-positive cocci. Haemophilus influenzae (Choice C) can also cause meningitis but is more commonly seen in unvaccinated children. Klebsiella pneumoniae (Choice D) is a Gram-negative bacterium but is not a typical causative agent of meningitis.

Correct Answer: B

Rationale: The correct answer is B: Thermophiles. Thermophiles are bacteria that can survive in extreme temperatures, typically between 45°C to 80°C. This ability is due to their unique enzymes and cell membrane structures that are adapted to high temperatures. Escherichia coli (A) is a common bacterium found in the human gut and does not thrive in extreme temperatures. Clostridium tetani (C) is a bacterium that causes tetanus and prefers moderate temperatures. Salmonella enterica (D) is a pathogenic bacterium that does not thrive in extreme temperatures. Therefore, Thermophiles are the only bacteria on the list known for surviving in extreme temperatures.

Correct Answer: B

Rationale: The correct answer is B because the AST reaction, or anti-streptolysin titer, is used in the diagnostic of post-streptococcal sequelae, such as rheumatic fever and glomerulonephritis. This test measures the level of antibodies produced by the body in response to Streptolysin O, a toxin produced by Group A Streptococcus bacteria. It helps in identifying if a patient has had a recent Streptococcus infection that could lead to these complications. A: Incorrect. The AST reaction does not require urine samples, but rather blood samples. C: Incorrect. The AST reaction specifically targets antibodies produced in response to Group A Streptococcus. D: Incorrect. The AST reaction does not directly identify isolated streptococci but rather measures the body's immune response to Streptolysin O.

Correct Answer: C

Rationale: The correct answer is C because both Mycobacterium bovis and Mycobacterium tuberculosis are bacteria that can cause tuberculosis in humans. Mycobacterium bovis primarily infects animals but can be transmitted to humans, while Mycobacterium tuberculosis is the main causative agent of tuberculosis in humans. Therefore, both choices A and B are correct. Choice D is incorrect because we know that Mycobacterium bovis and Mycobacterium tuberculosis are causative agents of tuberculosis in humans.

Correct Answer: D

Rationale: The correct answer is D because the female reproductive system consists of the uterus, ovaries, vagina, and fallopian tubes. The ovaries produce eggs, the fallopian tubes transport the eggs to the uterus, where fertilization can occur. The vagina is the birth canal and where sperm is deposited during intercourse. The uterus is where a fertilized egg implants and a fetus develops. Choice A is incorrect because it includes the kidneys, which are part of the urinary system, not the reproductive system. Choice B is incorrect because it includes the bladder and duodenum, which are part of the urinary and digestive systems, respectively, not the reproductive system. Choice C is incorrect because it includes the bladder and ureter, which are part of the urinary system, not the reproductive system.

Correct Answer: B

Rationale: The correct answer is B because the presence of calcium dipicolinate in bacterial spores helps protect their DNA from damage under harsh conditions. Calcium dipicolinate acts as a stabilizer, preserving the spore structure. Choice A is incorrect as bacterial spores have a tough protein coat, not a lipid-rich membrane. Choice C is incorrect as bacterial spores have very low metabolic activity. Choice D is incorrect as bacterial spores have low water content to prevent damage during harsh conditions.

Correct Answer: A

Rationale: The correct answer is A: Pseudomonas aeruginosa. Pseudomonas aeruginosa is known for producing a blue-green pigment called pyocyanin, which is characteristic of its colonies. This bacterium is commonly associated with purulent infections, especially in immunocompromised patients. Proteus vulgaris (B) is not typically associated with blue-green pigment production. Bacillus subtilis (C) is not a common pathogen in wound infections. Klebsiella pneumoniae (D) does not produce blue-green pigment and is more commonly associated with respiratory infections. Therefore, based on the clinical presentation and pigment production, Pseudomonas aeruginosa is the most likely causative agent in this case.

Correct Answer: A

Rationale: The correct answer is A: Icosahedral structures. Icosahedral symmetry is commonly found in viruses due to its efficient way of packing genetic material. The icosahedron has 20 equilateral triangular faces, making it ideal for viral capsids. The other choices, B: Hexahedral, C: Tetrahedral, and D: Dodecahedral structures, do not accurately describe the cubic type of symmetry seen in viruses. Hexahedral refers to a six-faced polyhedron, tetrahedral to a four-faced polyhedron, and dodecahedral to a twelve-faced polyhedron, none of which align with the structure commonly observed in viruses.

Correct Answer: D

Rationale: The bacitracin sensitivity test is used to differentiate between different types of streptococci. Group A streptococci are sensitive to bacitracin, which means they will show inhibition around the bacitracin disk. This sensitivity is a characteristic feature of group A streptococci and helps in their identification. Staphylococci, pneumococci, and intestinal bacteria do not exhibit this sensitivity to bacitracin. Therefore, the correct answer is D: group A streptococci.