Gram-positive cocci arranged in chains were found in a wound smear. The bacteria were catalase-negative and beta-hemolytic on blood agar. What is the causative agent?

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pyogenes. This bacterium fits all the given characteristics: Gram-positive cocci arranged in chains, catalase-negative, and beta-hemolytic on blood agar. Streptococcus pyogenes is known for causing skin and soft tissue infections. Staphylococcus aureus (B) is catalase-positive and typically forms clusters, not chains. Enterococcus faecalis (C) is catalase-negative but not beta-hemolytic. Micrococcus luteus (D) is catalase-positive and not typically associated with wound infections.

Correct Answer: A

Rationale: The correct diagnosis is A: Balantidiasis. The presence of large ciliated protozoa with a kidney-shaped macronucleus is characteristic of Balantidium coli, the causative agent of balantidiasis. The ciliated trophozoites of Balantidium coli are unique compared to the other choices. Giardiasis is caused by Giardia lamblia, which has flagella and not cilia. Amoebiasis is caused by Entamoeba histolytica, which lacks cilia. Trichomoniasis is caused by Trichomonas vaginalis, which also lacks cilia. Therefore, based on the specific morphology described, the correct diagnosis is Balantidiasis.

Correct Answer: B

Rationale: The correct answer is B: Capsule. Capsules are a protective layer outside the bacterial cell wall that helps bacteria evade phagocytosis by immune cells. The capsule inhibits phagocytic cells from engulfing the bacteria, making it difficult for them to be recognized and destroyed. A: Flagella - Flagella are not involved in protecting bacteria from phagocytosis. They are responsible for bacterial movement. C: Endospore - Endospores are dormant, resistant structures formed by some bacteria in response to harsh conditions. They do not directly protect against phagocytosis. D: Pili - Pili are hair-like structures used for adhesion and conjugation, not for protection against phagocytosis.

Correct Answer: A

Rationale: The correct answer is A: Cell wall. Mycoplasma species lack a cell wall, making them unique among bacteria. This is because they have a flexible cell membrane that provides structural support instead of a rigid cell wall. The absence of a cell wall allows them to be pleomorphic and gives them resistance to certain antibiotics targeting cell wall synthesis. Plasma membrane (B), DNA (C), and ribosomes (D) are present in Mycoplasma species, so they are not the correct answer.

Correct Answer: B

Rationale: The correct answer is B: eukaryotic. Eukaryotic cells have a well-defined nucleus that encloses the genetic material. The nucleus is a distinguishing feature of eukaryotic cells, separating them from prokaryotic cells (choice A) which lack a nucleus. Anucleate cells (choice C) are cells that do not have a nucleus at all. Haploid cells (choice D) contain a single set of chromosomes, but this does not necessarily imply the presence of a nucleus. Therefore, the presence of a nucleus is what categorizes cells as eukaryotic.