A patient with pneumonia had a sputum smear stained by the Gram method revealing Gram-positive cocci in pairs. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. The presence of Gram-positive cocci in pairs indicates a bacterial morphology consistent with Streptococcus pneumoniae, a common cause of pneumonia. This bacterium is known to appear in pairs or short chains on Gram stain. Staphylococcus aureus (B) typically presents as clusters of Gram-positive cocci, Neisseria meningitidis (C) is a Gram-negative diplococcus, and Klebsiella pneumoniae (D) is a Gram-negative rod, making them less likely causative agents in this scenario.

Correct Answer: A

Rationale: The correct answer is A: Meningitides. H. Influenzae type b (Hib) is a bacterium known to cause meningitis, a serious infection of the protective membranes covering the brain and spinal cord. Hib does not typically cause tonsillitis, urinary tract infections, or gastroenteritis. Meningitis is a life-threatening condition that requires prompt treatment, making it crucial to identify Hib as the causative agent.

Correct Answer: B

Rationale: The correct answer is B: Streptococcus mutans. This bacterium is associated with the formation of dental caries due to its ability to metabolize sugars and produce acids that demineralize tooth enamel. Streptococcus pneumoniae (choice A) is not typically associated with dental caries. Bacillus anthracis (choice C) causes anthrax, not dental caries. Clostridium botulinum (choice D) causes botulism, not dental caries. Streptococcus mutans' unique characteristics make it the correct choice for the bacteria associated with dental caries.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. The presence of Gram-positive cocci in pairs indicates a bacterial morphology consistent with Streptococcus pneumoniae, a common cause of pneumonia. This bacterium is known to appear in pairs or short chains on Gram stain. Staphylococcus aureus (B) typically presents as clusters of Gram-positive cocci, Neisseria meningitidis (C) is a Gram-negative diplococcus, and Klebsiella pneumoniae (D) is a Gram-negative rod, making them less likely causative agents in this scenario.

Correct Answer: D

Rationale: The correct answer is D: stereomicroscopes. These types of microscopes, also known as dissecting microscopes, are designed for observing larger objects like insects or worms. They have a lower magnification power compared to electron microscopes (A), which are used to observe smaller objects at a much higher resolution. Dark-field microscopes (B) are used for observing live and unstained samples with high contrast, not specifically for larger objects. Fluorescence microscopes (C) are used to observe specimens that fluoresce under specific wavelengths of light, not necessarily for large objects like insects or worms. Therefore, the most suitable choice for observing fairly large objects like insects or worms is stereomicroscopes.

Correct Answer:

Rationale: Correct Answer: C: opportunists Rationale: 1. Opportunistic microorganisms cause infections only when the host's immune system is compromised. 2. They are typically harmless in healthy individuals but can become pathogenic in weakened hosts. 3. Saprophytic microorganisms are environmental decomposers, not inherently pathogenic. 4. Facultative pathogens can cause disease in both healthy and compromised hosts. 5. Obligate pathogens always cause disease regardless of the host's condition. Summary: A: Facultative pathogens can cause disease in healthy hosts. B: Saprophytes are not inherently pathogenic. D: Obligate pathogens always cause disease.

Correct Answer: A

Rationale: The correct answer is A: Shigella dysenteriae. It is the likely causative agent because it is a non-lactose-fermenting, Gram-negative rod commonly associated with severe diarrhea. Shigella dysenteriae specifically causes bacillary dysentery. Other choices are incorrect because: B: Escherichia coli is a lactose-fermenting bacterium commonly found in the gut and not typically associated with severe diarrhea. C: Salmonella typhi causes typhoid fever, not severe diarrhea. D: Proteus mirabilis is a urease-positive bacterium associated with urinary tract infections, not severe diarrhea.

Correct Answer: C

Rationale: The correct answer is C (60 years) because as individuals age, their immune system weakens, making them more susceptible to infectious diseases. Around the age of 60, people are more likely to succumb to infections, leading to a higher mortality rate. Choices A, B, and D are incorrect because they are younger ages, and the likelihood of infectious diseases causing a significant proportion of deaths increases with age.

Correct Answer: A

Rationale: The correct answer is A: Francisella tularensis. This bacterium is classified as a class A bioterrorism agent due to its high virulence and potential for mass casualties. It can cause tularemia, a severe and potentially fatal disease. The other choices, Hepatitis B, E. coli, and Bacillus subtilis, are not considered class A bioterrorism agents. Hepatitis B is a virus causing liver inflammation, E. coli is a common bacterium found in the gut, and Bacillus subtilis is a beneficial bacterium used in various industries. Therefore, Francisella tularensis is the correct choice based on its high pathogenicity and bioterrorism potential.