A child with diarrhea had stool microscopy revealing flagellated protozoa with two nuclei. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Giardia lamblia. Giardia is a flagellated protozoa with two nuclei, causing diarrhea. It is commonly found in contaminated water sources. Entamoeba histolytica (B) causes amoebic dysentery with trophozoites in stool. Trichomonas vaginalis (C) causes sexually transmitted infections, not diarrhea. Balantidium coli (D) is a ciliated protozoa causing dysentery with ciliated trophozoites. Therefore, Giardia lamblia is the most likely causative agent in this case.

Correct Answer: D

Rationale: The correct answer is D. Staphylococcus aureus, Escherichia coli, and Clostridium difficile are all known for causing hospital-acquired infections. Staphylococcus aureus can cause skin infections and pneumonia. Escherichia coli can lead to urinary tract infections. Clostridium difficile is associated with gastrointestinal infections. Each of these bacteria has been linked to healthcare-associated infections due to their ability to spread in healthcare settings. Therefore, the correct answer is D. Choices A, B, and C are incorrect because each of these bacteria individually is known to cause hospital-acquired infections.

Correct Answer: B

Rationale: The correct answer is B: Mycobacterium tuberculosis. This bacterium is specifically associated with tuberculosis, a contagious disease that primarily affects the lungs. Mycobacterium leprae (choice A) is associated with leprosy, not tuberculosis. Neisseria gonorrhoeae (choice C) causes gonorrhea, a sexually transmitted infection. Streptococcus pneumoniae (choice D) is known for causing pneumonia, not tuberculosis. Therefore, based on the association with the disease tuberculosis, the correct answer is Mycobacterium tuberculosis.

Correct Answer: D

Rationale: Step 1: Streptococcus pneumoniae is a common cause of community-acquired pneumonia. Step 2: Mycobacterium tuberculosis can lead to tuberculosis, which can manifest as pulmonary tuberculosis. Step 3: Escherichia coli is not typically associated with causing pneumonia. Step 4: Since Streptococcus pneumoniae and Mycobacterium tuberculosis are linked to pneumonia, and Escherichia coli is not, the correct answer is D - All of the above.

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.