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.

Correct Answer: C

Rationale: Rationale: 1. Infectious process involves interactions between macro and microorganisms. 2. It is not limited to manifested diseases or typical symptoms. 3. The process can occur without leading to a full-blown infectious disease. 4. Choice C correctly captures the dynamic relationship between different organisms. Summary: - A: Incorrect - Infectious process is not solely about clinically manifested diseases. - B: Incorrect - Not all infectious diseases have typical symptoms. - D: Incorrect - Ambulatory form is a specific type, not encompassing all infectious processes.

Correct Answer: D

Rationale: The correct answer is D: All of the above. 1. Escherichia coli can produce endotoxin due to its lipopolysaccharide component. 2. Salmonella enterica also produces endotoxin as part of its cell wall structure. 3. Pseudomonas aeruginosa is known to produce endotoxin as well. Therefore, all of these bacteria are capable of producing endotoxin, which can trigger an excessive immune response leading to septic shock. Other choices are incorrect because they do not cover all the bacteria mentioned in the question that are capable of producing endotoxin.

Correct Answer: A

Rationale: The correct answer is A: Streptomycin. Streptomycin is an aminoglycoside antibiotic used to treat tuberculosis that can cause hearing loss as a side effect. It is known to damage the cochlear and vestibular organs in the inner ear. The other choices, B: Isoniazid, C: Rifampicin, and D: Ethionamide, are not associated with causing hearing deterioration as a side effect. Isoniazid can rarely cause peripheral neuropathy, Rifampicin can lead to hepatotoxicity, and Ethionamide may cause gastrointestinal disturbances, but none of these drugs are known to specifically cause hearing loss.

Correct Answer: D

Rationale: The correct answer is D: chemoheterotrophs. Chemoheterotrophs use organic compounds as both their carbon source and energy source. They obtain energy by breaking down organic molecules through cellular respiration. Photoautotrophs (A) use light energy to produce organic compounds, chemoautotrophs (B) use inorganic compounds for energy, and photoheterotrophs (C) use light energy but require organic compounds as a carbon source. Therefore, chemoheterotrophs best describe bacteria that obtain both carbon and energy from organic compounds.

Correct Answer: B

Rationale: The optochin sensitivity test is used to differentiate Pneumococcal (Streptococcus pneumoniae) from other streptococci. Optochin sensitivity is a characteristic feature of Pneumococcal, as it inhibits its growth. The test involves placing optochin discs on a culture plate and observing the zone of inhibition around the disc for Pneumococcal. Neisseria gonorrhoeae, Staphylococci, and Group A streptococci do not exhibit optochin sensitivity, making them incorrect choices. In summary, the optochin sensitivity test is specific to Pneumococcal, making choice B the correct answer.

Correct Answer: B

Rationale: The correct answer is B: prophage. In the repressed stage, a phage integrates its DNA into the host cell's chromosome, becoming a prophage. This integration allows the phage to replicate along with the host cell without causing immediate lysis. A: Antiphage is not a term used to describe a phage in a repressed stage. C: Virulent phage refers to a phage that immediately causes lysis of the host cell. D: Latent phage is not a commonly used term in this context.

Correct Answer: A

Rationale: The correct answer is A: Entamoeba gingivalis. Firstly, it is important to note that Entamoeba gingivalis is commonly found in the oral cavity, specifically in carious cavities. Secondly, Entamoeba gingivalis belongs to the Sarcodina class, which is characterized by their amoeboid movement using pseudopods. This protozoa is associated with oral health issues, particularly in individuals with poor oral hygiene. Now, let's analyze why the other choices are incorrect: B: Entamoeba histolutica is known for causing amoebic dysentery, not commonly associated with oral cavities. C: Entamoeba coli is typically found in the human large intestine, not in oral cavities. D: Amoeba proteus is a free-living amoeba commonly used in laboratory studies and not typically associated with oral cavity infections.

Correct Answer: A

Rationale: The correct answer is A: McConkey agar. Selective media selectively inhibit the growth of certain organisms while allowing others to grow. McConkey agar contains crystal violet and bile salts that inhibit the growth of Gram-positive bacteria, making it selective for Gram-negative bacteria. Choices B and C are not selective media, as they do not specifically inhibit the growth of certain types of bacteria. Choice D is incorrect as McConkey agar is indeed an example of selective media.