A fecal smear from a patient with chronic diarrhea revealed cysts with four nuclei. What is the causative agent?

Correct Answer: A

Rationale: The correct answer is A: Entamoeba histolytica. The presence of cysts with four nuclei is characteristic of Entamoeba histolytica, a protozoan parasite causing amoebic dysentery. It forms cysts with four nuclei before maturing into trophozoites. Balantidium coli (B) is a ciliated protozoan causing balantidiasis, not known for four-nuclei cysts. Giardia lamblia (C) forms cysts with two nuclei, causing giardiasis. Trichomonas hominis (D) is a non-pathogenic flagellate found in the human colon, not associated with four-nuclei cysts.

Correct Answer: A

Rationale: The correct answer is A: Klebsiella pneumoniae. This bacterium is commonly associated with pneumonia in elderly patients. Klebsiella pneumoniae is a Gram-negative, non-motile, rod-shaped bacterium that frequently causes pneumonia in individuals with underlying health conditions. It is known for its ability to produce a distinctive mucoid capsule. Pseudomonas aeruginosa (B) is more commonly associated with hospital-acquired infections, particularly in immunocompromised individuals. Escherichia coli (C) is a common cause of urinary tract infections but is less likely in this case due to the symptoms and sputum sample results. Haemophilus influenzae (D) is another common respiratory pathogen, but in this scenario, Klebsiella pneumoniae is the most likely causative agent based on the patient's age, symptoms, and the characteristics of the bacteria described in the sputum sample.

Correct Answer: B

Rationale: The correct answer is B: biofilms. Biofilms are structured communities of microorganisms that adhere to a surface and are embedded in a matrix. This is the most appropriate term as it specifically describes the organization of various species into complex communities on a surface. Colonies (A) refer to groups of cells of the same species, not necessarily multiple species. Ecosystems (C) are broader systems involving interactions between living organisms and their environment. Plagues (D) are outbreaks of disease, not a term used to describe organized microbial communities.

Correct Answer: D

Rationale: The correct answer is D: Proteus vulgaris. Non-lactose-fermenting, Gram-negative rods that produce gas in a nutrient broth are characteristic of Proteus species. Proteus vulgaris is known for its swarming motility and ability to produce hydrogen sulfide gas. Salmonella typhi and Shigella dysenteriae are Enterobacteriaceae that do not produce gas in a nutrient broth. Escherichia coli is a lactose-fermenting bacterium and would not be expected to produce gas in a nutrient broth.

Correct Answer: A

Rationale: The correct answer is A: Borrelia burgdorferi. This bacterium is known for causing Lyme disease, which is transmitted through tick bites. Borrelia burgdorferi is specifically associated with Lyme disease due to its ability to infect and spread within the body, leading to a range of symptoms if left untreated. Summary of incorrect choices: B: Clostridium difficile - Known for causing gastrointestinal infections, not Lyme disease. C: Streptococcus pneumoniae - Commonly causes pneumonia and other respiratory infections, not Lyme disease. D: Mycobacterium tuberculosis - Causes tuberculosis, a respiratory disease, not Lyme disease.