On examination of a 6-year-old child the doctor noticed greyish film on the child's tonsils. Microscopy of the smear stained by Neisser method detected there Corynebacterium diphtheria. What morphologic feature was the most indicative for determining the type of the agent?

Correct Answer: C

Rationale: The correct answer is C: Polar placement of volutin granules. This feature is indicative of Corynebacterium diphtheria. Volutin granules are metachromatic granules found in the polar region of the bacterial cell, which is characteristic of this species. This feature helps differentiate it from other bacteria. Explanation: A: Fence-like position of the agent's cells is not a specific morphologic feature of Corynebacterium diphtheria. B: Spores that exceed cells in diameter is not a characteristic feature of Corynebacterium diphtheria. D: Presence of the capsule is not a distinguishing morphologic feature of Corynebacterium diphtheria. In summary, the polar placement of volutin granules is the most indicative feature for determining the type of agent as Corynebacterium diphtheria, making it the correct choice.

Correct Answer: B

Rationale: The correct answer is B because the presence of calcium dipicolinate in bacterial spores helps protect their DNA from damage under harsh conditions. Calcium dipicolinate acts as a stabilizer, preserving the spore structure. Choice A is incorrect as bacterial spores have a tough protein coat, not a lipid-rich membrane. Choice C is incorrect as bacterial spores have very low metabolic activity. Choice D is incorrect as bacterial spores have low water content to prevent damage during harsh conditions.

Correct Answer: C

Rationale: The correct answer is C: ensure survival under harsh conditions. Bacterial endospores are dormant, resistant structures that form under adverse conditions to protect the bacterial DNA and allow the bacterium to survive extreme environments. This function is crucial for the bacterium's long-term survival. Choice A is incorrect as endospores do not directly produce exotoxins. Choice B is incorrect as endospores do not store nutrients but rather protect the bacterium's genetic material. Choice D is incorrect as endospores do not enhance motility but rather enable survival during unfavorable conditions.

Correct Answer: B

Rationale: The correct answer is B: peptidoglycan and calcium dipicolinate. Bacterial endospores are resistant structures formed by certain bacterial species under unfavorable conditions. Peptidoglycan provides structural support, while calcium dipicolinate aids in heat resistance. Lipid bilayers (choice A) are not a primary component. Capsular polysaccharides (choice C) are part of the bacterial capsule, not the endospore. Teichoic acid (choice D) is a component of the cell wall, not the endospore.

Correct Answer: A

Rationale: The correct answer is A: Trichomonas vaginalis. Trichomonas vaginalis is a sexually transmitted protozoan that causes trichomoniasis. The pear-shaped flagellated protozoa with an undulating membrane described in the question match the morphology of Trichomonas vaginalis. It is typically found in vaginal discharge of infected individuals. Giardia lamblia (B) is a different protozoan that causes giardiasis and has a distinct morphology of a teardrop shape with flagella. Trypanosoma brucei (C) causes African sleeping sickness and Leishmania donovani (D) causes visceral leishmaniasis, both of which have different morphologies and clinical presentations compared to Trichomonas vaginalis.