Animals that are capable of transmitting infectious disease are called:

Correct Answer: A

Rationale: The correct answer is A: vectors. Vectors are animals that transmit infectious diseases from one host to another, typically through biting or contact. They do not cause the disease themselves but serve as intermediaries. Pathogens (B) are the microorganisms that cause diseases. Carriers (C) are individuals who harbor a pathogen without showing symptoms. Parasites (D) are organisms that live on or inside another organism and may or may not transmit diseases. Therefore, the term "vectors" specifically refers to animals that transmit infectious diseases, making it the correct choice in this context.

Correct Answer: D

Rationale: The correct answer is D because all three viruses, Vaccinia virus, Smallpox virus, and Cowpox virus, belong to the genus Orthopoxvirus. They share similar characteristics and genetic makeup. Vaccinia virus is used as a vaccine against smallpox, Smallpox virus causes smallpox disease, and Cowpox virus can infect both animals and humans. Choice A, B, and C are incorrect because each of these viruses is a member of the Orthopoxvirus genus.

Correct Answer: A

Rationale: The correct answer is A: Acid-fast stain. This stain is used for diagnosing tuberculosis due to the unique cell wall composition of Mycobacterium tuberculosis, which resists decolorization by acid-alcohol. This allows the bacteria to retain the primary stain (carbol fuchsin) and appear red under a microscope. Gram stain (B) is not suitable for Mycobacterium tuberculosis as it is gram-positive but does not retain the crystal violet-iodine complex. Negative stain (C) is not used for bacterial identification. Simple stain (D) does not differentiate acid-fast bacteria like Mycobacterium tuberculosis.

Correct Answer: C

Rationale: The correct answer is C: Helicobacter pylori. Group I organisms are classified based on their low infectious doses and potential for bioterrorism. H. pylori is known to cause gastric ulcers and is categorized as a Group I organism due to its ability to cause disease at low doses. A: Francisella tularensis is a Group A bioterrorism agent, not Group I. B: Clostridium difficile is not typically classified as a Group I organism. D: Bacillus anthracis is a Group A bioterrorism agent, not Group I.

Correct Answer: A

Rationale: The correct answer is A: Treponema. Treponema pallidum is a spirochete bacterium that appears as long, thin, and spiral-shaped microorganisms with 8-12 regular coils under the microscope. This characteristic morphology is specific to Treponema species, making it the correct choice. Borrelia, Leptospira, and Vibrios have different shapes and arrangements under the microscope, making them incorrect choices in this scenario.

Correct Answer: C

Rationale: The correct answer is C: Gram-negative bacterial outer membrane. Lipopolysaccharides (LPS) are major components of the outer membrane of Gram-negative bacteria. LPS consist of lipid A, core oligosaccharide, and O-antigen. They play a crucial role in the structural integrity and protection of Gram-negative bacteria. Choice A (Gram-positive bacteria) is incorrect because LPS are not present in Gram-positive bacteria. Choice B (Fungal cell walls) is incorrect as LPS are specific to bacterial outer membranes. Choice D (Viral capsids) is incorrect as LPS are not associated with viral structures.

Correct Answer: A

Rationale: The correct answer is A: Giardia lamblia. Giardia lamblia is a flagellated protozoan with two nuclei, known as the "falling leaf" appearance. It causes diarrhea in humans through ingestion of contaminated water or food. Entamoeba histolytica causes amoebic dysentery, characterized by bloody diarrhea and colitis. Trichomonas vaginalis is a sexually transmitted protozoan causing vaginal discharge but is not found in stool. Balantidium coli is a ciliated protozoan causing diarrhea but does not have flagellated forms.

Correct Answer: A

Rationale: Step 1: Streptococcus mutans is known for its ability to produce lactic acid, which contributes to the demineralization of tooth enamel, leading to dental caries. Step 2: Streptococcus pneumoniae is a common respiratory pathogen and not directly associated with dental caries. Step 3: Bacillus cereus is a foodborne pathogen and not typically found in the oral cavity or associated with dental caries. Step 4: Clostridium tetani causes tetanus, a neurological condition, and is not involved in the formation of dental caries. Summary: Choice A is correct as Streptococcus mutans is specifically linked to dental caries due to its acidogenic and aciduric properties. Choices B, C, and D are incorrect as they are not associated with dental caries.

Correct Answer: C

Rationale: The correct answer is C: Thermophiles. Thermophiles are bacteria that thrive in high temperatures typically above 45°C. They have specialized enzymes and cellular structures that allow them to survive and even thrive in extreme heat. Psychrophiles (A) are adapted to cold temperatures, mesophiles (B) thrive at moderate temperatures, and halophiles (D) prefer high salt concentrations, making them unsuitable for extreme heat survival. Therefore, Thermophiles are the only bacteria among the choices that are known for their ability to survive in extreme heat.