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: D

Rationale: The correct answer is D. Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae are all known to cause meningitis. Neisseria meningitidis is a common cause of bacterial meningitis in young adults. Haemophilus influenzae is a leading cause of meningitis in children, and Streptococcus pneumoniae is a common cause in adults. Therefore, all three bacteria are responsible for causing meningitis, making option D the correct choice. Option A, B, and C are incorrect because they each represent only one of the bacteria responsible for causing meningitis, while option D includes all three.

Correct Answer: D

Rationale: Step-by-step rationale for why answer D is correct: 1. Tetanus exotoxins are produced by Clostridium tetani bacteria. 2. Once formed, the exotoxins can spread through the bloodstream. 3. Retrograde axonal transport allows the exotoxins to travel to the central nervous system (CNS). 4. In the CNS, the exotoxins can cause the symptoms of tetanus. 5. Options A, B, and C are incorrect as tetanus exotoxins do not follow those pathways. In summary, option D is correct as tetanus exotoxins can spread through the blood and reach the CNS via retrograde axonal transport, while the other options do not align with the known pathophysiology of tetanus.

Correct Answer: A

Rationale: The correct answer is A: Clostridium tetani. The presence of Gram-positive rods with terminal spores resembling drumsticks indicates Clostridium tetani, the causative agent of tetanus. This bacterium forms spores at one end, giving it a drumstick appearance under the microscope. Clostridium perfringens (B) and Clostridium botulinum (D) do not have terminal spores. Bacillus anthracis (C) forms spores but typically appears as a square-ended or bamboo stick shape. Therefore, based on the microscopy findings, Clostridium tetani is the most likely microorganism responsible for the wound infection.

Correct Answer: A

Rationale: The correct answer is A: spleen. The spleen is not a component of the gastrointestinal tract; it is part of the lymphatic system. The pharynx, esophagus, and stomach are all part of the gastrointestinal tract responsible for digestion and absorption of food. The spleen's main functions include filtering blood, storing blood cells, and supporting the immune system, making it unrelated to the digestive process.

Correct Answer: B

Rationale: The correct answer is B because peritrichous bacteria have flagella distributed all over their surface, allowing them to move in various directions. Choice A is incorrect because bacteria with a single flagellum at one pole are called monotrichous. Choice C is incorrect as bacteria with a single flagellum at each pole are known as amphitrichous. Choice D is incorrect because peritrichous bacteria have multiple flagella all over their surface, not limited to one or both poles.

Correct Answer: A

Rationale: The correct answer is A: Salk and Sabin vaccines are vaccines against polio. Both Salk and Sabin vaccines are indeed vaccines specifically developed to prevent polio. The Salk vaccine is an inactivated polio vaccine, while the Sabin vaccine is an oral live attenuated vaccine. Therefore, choice B is incorrect as the Sabin vaccine is live attenuated, not inactivated. Choice C is also incorrect because the Salk vaccine is inactivated, not live attenuated. Choice D is incorrect because not all statements are true; only statement A is accurate.

Correct Answer: A

Rationale: The correct answer is A: Haemolytic streptococci. Hemolytic streptococci are known to cause serious infections in humans, including strep throat and skin infections. These bacteria have the potential to spread rapidly and cause epidemics. Bacilli are a broad category of bacteria and not all are necessarily harmful. Micrococci are commonly found on human skin and are generally harmless. Yeast fungi are also commonly found in the environment and are not typically associated with causing epidemics. Therefore, the presence of hemolytic streptococci in the air of a pharmacy indicates a direct epidemic danger due to their pathogenic nature and potential to cause widespread infections.

Correct Answer: D

Rationale: The correct answer is D: Yersinia enterocolitica. This bacterium can contaminate foods and grow at refrigerator temperatures due to its psychrotrophic nature. Yersinia enterocolitica is known to cause foodborne illnesses even when stored at cold temperatures. Shigella sonnei is a human pathogen transmitted through fecal-oral route, Campylobacter jejuni is typically found in poultry and causes food poisoning, and E. coli can cause foodborne illnesses but does not grow well at refrigerator temperatures.