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: C

Rationale: The correct answer is C: Bacillus subtilis. Bacillus subtilis is known for its spore-forming ability, which is a survival mechanism to withstand harsh conditions. Spore formation allows the bacteria to become dormant and resistant to heat, chemicals, and radiation. Escherichia coli (A) is not known for spore formation. Staphylococcus aureus (B) does not form spores either. Listeria monocytogenes (D) is a pathogenic bacterium but does not typically form spores.

Correct Answer: A

Rationale: The correct answer is A: Clostridium botulinum. This bacterium produces the neurotoxin responsible for botulism. It thrives in anaerobic environments like improperly canned foods. Clostridium difficile (B) causes colitis, not botulism. Staphylococcus aureus (C) causes food poisoning but not botulism. Escherichia coli (D) can cause gastrointestinal illness but not botulism.

Correct Answer: A

Rationale: The correct answer is A: Bacterial ribosomes synthesize proteins. Ribosomes are cellular organelles responsible for protein synthesis by translating mRNA into amino acids. They are essential for the production of proteins needed for cell function. Choices B, C, and D are incorrect because ribosomes do not synthesize DNA, degrade toxic substances, or assist in cell division. DNA synthesis is carried out by DNA polymerase, degradation of toxic substances is typically done by enzymes, and cell division is primarily regulated by the cell cycle machinery.

Correct Answer: B

Rationale: Correct Answer: B - Plasmids are circular DNA molecules Rationale: 1. Plasmids are extrachromosomal DNA molecules found in bacteria. 2. Plasmids are typically circular in structure, not linear. 3. Circular plasmids are easily replicated and transferred between bacteria. 4. Plasmids can carry genes for antibiotic resistance, making option C incorrect. 5. Not all bacteria have plasmids, making option D incorrect. 6. While plasmids can provide benefits, they are not essential for bacterial survival, making option A incorrect.