While examining the feces of a patient with diarrhea, cysts with four nuclei were found. What protozoan is most likely causing the condition?

Correct Answer: A

Rationale: The correct answer is A: Entamoeba histolytica. This protozoan causes amoebiasis, which presents with symptoms like diarrhea. The cysts with four nuclei found in the feces are characteristic of Entamoeba histolytica. This protozoan is known to cause invasive infection, leading to tissue destruction and ulcers in the intestines. Lamblia (choice B) typically presents with cysts containing two nuclei. Trichomonas (choice C) is a sexually transmitted parasite and does not form cysts. Balantidium (choice D) is a ciliated protozoan that does not have four nuclei in its cyst form.

Correct Answer: A

Rationale: The correct answer is A: axial filaments. Treponemae are motile due to their unique structure of axial filaments, also known as endoflagella. These axial filaments are located within the periplasmic space of the organism and are responsible for the corkscrew-like motility of Treponemae. The other choices (B: mans peritrichous filaments, C: single monotrichous filament, D: fimbriae) do not play a role in the motility of Treponemae. Peritrichous filaments and single monotrichous filaments are not present in Treponemae, and fimbriae are primarily involved in adhesion rather than motility.

Correct Answer: B

Rationale: The correct answer is B: protein synthesis. Bacterial ribosomes are responsible for translating messenger RNA into proteins through a process called translation. Ribosomes are composed of both protein and RNA molecules and facilitate the binding of transfer RNA to the messenger RNA to assemble the amino acids in the correct order to form a protein. DNA replication (A) is carried out by enzymes such as DNA polymerase, not ribosomes. Cell wall synthesis (C) is a process involving enzymes that build the structure of the cell wall in bacteria. RNA splicing (D) is a eukaryotic process that involves removing introns from pre-mRNA molecules, which is not a function of bacterial ribosomes.

Correct Answer: B

Rationale: The correct answer is B: resisting phagocytosis. Bacterial capsules are composed of polysaccharides that help bacteria evade phagocytosis by immune cells. This physical barrier prevents immune cells from engulfing and destroying the bacteria. Facilitating motility (A), synthesizing DNA (C), and producing energy (D) are not primary functions of bacterial capsules. Motility is usually facilitated by flagella, DNA synthesis occurs in the bacterial chromosome, and energy production is typically carried out by metabolic processes within the bacterial cell.

Correct Answer: B

Rationale: The correct answer is B: Peptic ulcer. Peptic ulcers are commonly caused by Helicobacter pylori bacteria, which can be transmitted through the fecal-oral route. This means that the bacteria from infected feces can be ingested through contaminated food or water, leading to the development of peptic ulcers in the gastrointestinal tract. Incorrect choices: A: Ehrlichiosis - Ehrlichiosis is a tick-borne disease transmitted through the bite of an infected tick, not through the fecal-oral route. C: Lyme disease - Lyme disease is also transmitted through the bite of an infected tick, specifically the black-legged tick. D: Tularemia - Tularemia is a bacterial disease typically transmitted through insect bites, handling of infected animals, or ingestion of contaminated water, not through the fecal-oral route.

Correct Answer: A

Rationale: The correct answer is A: evade host immune responses. Antigenic variation allows bacteria to change their surface antigens, making it difficult for the host immune system to recognize and attack them effectively. This enhances the bacteria's ability to evade immune responses and establish infection. B: Increase antibiotic susceptibility is incorrect because antigenic variation does not directly impact the susceptibility of bacteria to antibiotics. C: Increase peptidoglycan synthesis is incorrect because antigenic variation is not related to the synthesis of peptidoglycan, which is a major component of the bacterial cell wall. D: Enhance motility is incorrect because antigenic variation does not directly affect the motility of bacteria. In summary, antigenic variation in bacteria primarily serves to help them evade host immune responses, making option A the correct answer.

Correct Answer: B

Rationale: The correct answer is B: Rhodophyta. Rhodophyta is the phylum of red algae, which are known to contain agar in their cell walls. Agar is a gelatinous substance derived from these algae. Chrysophyta (A), Chlorophyta (C), and Phaeophyta (D) do not typically contain agar in their cell walls. Chrysophyta includes diatoms, Chlorophyta includes green algae, and Phaeophyta includes brown algae, none of which are known for agar production. Therefore, Rhodophyta is the correct choice based on the presence of agar in their cell walls.

Correct Answer: A

Rationale: The presence of rods with polar dark blue granules stained by Neisser's method indicates metachromatic granules consistent with Corynebacterium diphtheriae. The characteristic staining pattern is due to the presence of polar granules in the bacterial cells, a feature specific to Corynebacterium diphtheriae. Bacillus anthracis, Clostridium tetani, and Escherichia coli do not exhibit this staining pattern or morphology typical of Corynebacterium diphtheriae, making them incorrect choices.

Correct Answer: D

Rationale: Production of auto-antibodies can be due to various factors. Mutant B cell clones can lead to the production of self-reactive antibodies. Antibodies against hidden tissues can also trigger autoimmune responses. Genetic factors play a crucial role in predisposing individuals to autoimmunity. Therefore, the correct answer is D, as all these factors can contribute to the production of auto-antibodies. Choice A is incorrect as it only addresses one possible cause. Choice B is incorrect as it focuses solely on antibodies against hidden tissues. Choice C is incorrect as it overlooks the other potential reasons for auto-antibody production.