Correct Answer: B

Rationale: The correct answer is B: Salmonellosis. The description of microorganisms resembling a tennis racket indicates the presence of Salmonella typhi, which causes salmonellosis. Salmonella bacteria can appear like tennis rackets under a microscope due to their flagella. Botulism (
A) is caused by Clostridium botulinum, which does not resemble a tennis racket. Cholera (
C) is caused by Vibrio cholerae, which appears as curved rods under the microscope. Dysentery (
D) is usually caused by bacteria like Shigella or Entamoeba histolytica, which do not resemble tennis rackets.

Correct Answer: D

Rationale: The correct answer is D: HIV. HIV is transmitted through direct contact with certain body fluids such as blood, semen, vaginal fluids, and breast milk. This direct contact route of transmission makes it different from the other choices. Salmonellosis is typically transmitted through contaminated food or water (not direct contact). Tularemia is usually transmitted through insect bites or contact with infected animals. Rubella is primarily transmitted through respiratory droplets.
Therefore, HIV is the only option with a contact route of transmission.

Correct Answer: A

Rationale: The correct answer is A: Icosahedral structures. Icosahedral symmetry is commonly found in viruses due to its efficient way of packing genetic material. The icosahedron has 20 equilateral triangular faces, making it ideal for viral capsids. The other choices, B: Hexahedral, C: Tetrahedral, and D: Dodecahedral structures, do not accurately describe the cubic type of symmetry seen in viruses. Hexahedral refers to a six-faced polyhedron, tetrahedral to a four-faced polyhedron, and dodecahedral to a twelve-faced polyhedron, none of which align with the structure commonly observed in viruses.

Correct Answer: C

Rationale: The correct answer is C: crateriform. In a gelatin stab, a saucer-shaped liquefaction indicates a depression or crater-like shape. This term accurately describes the appearance of the liquefaction.

A: napiform means turnip-shaped, which is not relevant to the shape in a gelatin stab.
B: saccate means pouch-like, which does not match the saucer-shaped description.
D: stratiform refers to something in layers, which is not applicable to the shape of the liquefaction.


Therefore, C is the most appropriate term to describe the saucer-shaped liquefaction in a gelatin stab.

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.