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 Amebiasis. The patient's symptoms of abdominal pain, loose stools with mucus and blood, along with the presence of protozoa containing phagocytosed erythrocytes in stool analysis are indicative of Entamoeba histolytica infection. The size (30-40 microns) and pseudopodia characteristics match E. histolytica. Amebiasis commonly presents with dysentery, while Leishmaniasis (B) typically causes skin ulcers. Trichomoniasis (C) is a sexually transmitted infection, not associated with the described symptoms. Giardiasis (D) is characterized by foul-smelling diarrhea and steatorrhea, not bloody diarrhea as seen in this case.

Correct Answer: A

Rationale: Step 1: Streptococcus pyogenes is commonly known as Group A Streptococcus, which is the primary bacteria responsible for causing strep throat. Step 2: Staphylococcus aureus is not typically associated with strep throat but can cause skin infections and other illnesses. Step 3: Neisseria gonorrhoeae is the bacteria responsible for gonorrhea, a sexually transmitted infection, not strep throat. Step 4: Clostridium tetani causes tetanus, a bacterial infection that affects the nervous system and is not related to strep throat.

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: 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.