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

Correct Answer: A

Rationale: The correct answer is A: Entamoeba histolytica. This protozoan parasite causes amoebic dysentery and is characterized by cysts with four nuclei in fecal smears. It is commonly associated with chronic diarrhea. Balantidium coli (B) presents with large ciliated trophozoites, not cysts. Giardia lamblia (C) has trophozoites with flagella and causes giardiasis, not cysts with four nuclei. Trichomonas hominis (D) typically has a pear-shaped trophozoite and is not associated with cysts with four nuclei.

Correct Answer: A

Rationale: The correct answer is A: Break down antibiotics like penicillin. Beta-lactamase is an enzyme produced by bacteria to inactivate beta-lactam antibiotics like penicillin by breaking the beta-lactam ring. This mechanism helps bacteria resist the antibiotic's effects. The other choices are incorrect because beta-lactamase does not enhance protein synthesis, facilitate DNA replication, or transport nutrients. It specifically targets and deactivates beta-lactam antibiotics, making choice A the most appropriate answer.

Correct Answer: C

Rationale: The correct answer is C: Enhancing virulence by preventing phagocytosis. Bacterial capsules help bacteria evade the immune system by preventing phagocytosis, a process where immune cells engulf and destroy pathogens. This enhances the bacteria's virulence and ability to cause infections. Choices A, B, and D are incorrect because bacterial capsules do not directly contribute to protein synthesis, protect against desiccation, or facilitate motility.

Correct Answer: A

Rationale: Pili in bacteria are responsible for conjugation, allowing the transfer of genetic material between cells. They form a bridge between donor and recipient cells, facilitating the exchange of DNA. Flagella are for movement, capsules for protection, and ribosomes for protein synthesis. Pili is the only structure directly involved in genetic material exchange.

Correct Answer: B

Rationale: The correct answer is B: Mycobacterium tuberculosis. Mycobacterium tuberculosis is the bacteria responsible for causing tuberculosis. This bacterium has unique cell wall components that make it resistant to the body's immune defenses, allowing it to survive and multiply within the host. Streptococcus pyogenes (Choice A), Escherichia coli (Choice C), and Bacillus anthracis (Choice D) are not associated with tuberculosis. Streptococcus pyogenes is known for causing strep throat and skin infections, Escherichia coli is commonly involved in gastrointestinal infections, and Bacillus anthracis causes anthrax.