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

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microbiology an introduction 12th edition test bank Questions

Question 1 of 5

A patient with chronic 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 lamblia is a flagellated protozoan with two nuclei found in the stool. It commonly causes chronic diarrhea. The other choices are incorrect because B: Entamoeba histolytica causes dysentery, not chronic diarrhea; C: Trichomonas vaginalis is a sexually transmitted infection, not typically found in stool; D: Balantidium coli is a ciliated protozoan, not flagellated.

Question 2 of 5

A patient with diarrhea had stool microscopy revealing large, oval protozoa with a single nucleus and four flagella. What is the most likely causative agent?

Correct Answer: A

Rationale: Step 1: Large, oval protozoa with a single nucleus and four flagella indicate Giardia lamblia. Step 2: Giardia lamblia is a flagellated protozoan causing diarrhea. Step 3: It has a distinct appearance under microscopy. Step 4: Entamoeba histolytica causes dysentery with no flagella. Step 5: Balantidium coli is a ciliated protozoan with a larger size. Step 6: Trichomonas vaginalis lacks a cyst stage and causes genitourinary infections.

Question 3 of 5

A patient with a productive cough and fever had a Gram-positive cocci isolated from a sputum sample. The bacteria were catalase-positive and coagulase-negative. What is the likely microorganism?

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus epidermidis. 1. Gram-positive cocci suggest a staphylococcal species. 2. Catalase-positive indicates it belongs to the Staphylococcus genus. 3. Coagulase-negative rules out Staphylococcus aureus. 4. Staphylococcus epidermidis is a common cause of infections in immunocompromised patients. Summary: - B: Staphylococcus aureus is coagulase-positive. - C: Streptococcus pneumoniae is a Gram-positive diplococci. - D: Neisseria meningitidis is a Gram-negative diplococci.

Question 4 of 5

Which bacteria produce a toxin that inhibits protein synthesis?

Correct Answer: A

Rationale: The correct answer is A: Shigella dysenteriae. This bacteria produces the Shiga toxin, which inhibits protein synthesis by interfering with the ribosomes. Shigella dysenteriae causes severe gastrointestinal illness. The other choices, B: Clostridium botulinum, produces a neurotoxin that blocks neurotransmitter release. C: Streptococcus pneumoniae causes pneumonia and other respiratory infections. D: Mycobacterium tuberculosis causes tuberculosis by infecting the lungs. Thus, based on the toxin production and mode of action, Shigella dysenteriae is the correct answer.

Question 5 of 5

A child with fever and swollen lymph nodes had a Gram-stained smear revealing Gram-negative rods with a safety pin appearance. What is the causative agent?

Correct Answer: A

Rationale: The correct answer is A: Yersinia pestis. This bacterium causes the plague, which presents with fever and swollen lymph nodes. The safety pin appearance on Gram stain is characteristic of Yersinia pestis. B: Francisella tularensis causes tularemia, which presents differently and does not show safety pin appearance on Gram stain. C: Brucella abortus causes brucellosis, which does not typically present with the same symptoms as described in the question. D: Clostridium botulinum causes botulism, which does not show safety pin appearance on Gram stain and does not typically present with fever and swollen lymph nodes.

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