Which of the following bacteria is known for causing dental caries (cavities)?
- A. Streptococcus mutans
- B. Streptococcus pneumoniae
- C. Bacillus anthracis
- D. Clostridium botulinum
Correct Answer: A
Rationale: Step 1: Streptococcus mutans is known for its ability to produce acids that contribute to the demineralization of tooth enamel, leading to dental caries.
Step 2: Streptococcus pneumoniae is a common cause of respiratory infections, not dental caries.
Step 3: Bacillus anthracis causes anthrax, a serious bacterial infection, not dental caries.
Step 4: Clostridium botulinum causes botulism, a severe form of food poisoning, not dental caries.
Summary: Streptococcus mutans is the correct answer as it is specifically associated with causing dental caries, while the other choices are linked to different diseases.
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For the structure of rubella virus, it is true that
- A. has icosahedral type of symmetry
- B. is ssDNA virus
- C. does not have an envelope
- D. does not have glycoprotein spikes
Correct Answer: A
Rationale: Step 1: Rubella virus belongs to the Togaviridae family, which typically exhibits icosahedral symmetry.
Step 2: Icosahedral symmetry refers to a structure with 20 equilateral triangular faces, which is characteristic of many viruses, including rubella.
Step 3: Therefore, choice A is correct as rubella virus indeed has icosahedral type of symmetry.
Summary:
- Choice B is incorrect as rubella virus is an ssRNA virus, not ssDNA.
- Choice C is incorrect as rubella virus does have an envelope derived from the host cell membrane.
- Choice D is incorrect as rubella virus does have glycoprotein spikes on its surface.
A fecal sample from a patient with diarrhea revealed cysts with four nuclei. What is the most likely causative agent?
- A. Entamoeba histolytica
- B. Giardia lamblia
- C. Trichomonas vaginalis
- D. Balantidium coli
Correct Answer: A
Rationale: The correct answer is A: Entamoeba histolytica. This is because the presence of cysts with four nuclei is characteristic of Entamoeba histolytica, a protozoan parasite that causes amoebic dysentery. Entamoeba histolytica cysts contain four nuclei, distinguishing them from other pathogens. Giardia lamblia and Trichomonas vaginalis are not associated with cysts containing four nuclei. Balantidium coli is a ciliated protozoan that does not typically form cysts with multiple nuclei. In summary, the unique feature of four nuclei in the cysts points towards Entamoeba histolytica as the most likely causative agent of the diarrhea in this patient.
Examination of the duodenal contents revealed some pear-shaped protozoa with two nuclei and four pairs of flagella. The organisms had also two axostyles between the nuclei and a ventral adhesive disc. What protozoan representative was found in the patient?
- A. Lamblia
- B. Toxoplasma
- C. Leishmania
- D. Intestinal trichomonad
Correct Answer: A
Rationale: Step 1: Pear-shaped protozoa with two nuclei, four pairs of flagella, two axostyles, and a ventral adhesive disc match the characteristics of Giardia lamblia.
Step 2: Giardia lamblia is commonly found in the duodenum and causes giardiasis.
Step 3: Lamblia is the correct answer as it specifically matches all the described characteristics.
Summary: B, Toxoplasma, is incorrect as it does not match the described features. C, Leishmania, does not have the specific characteristics mentioned. D, Intestinal trichomonad, does not match the detailed descriptions given.
A patient with a sore throat and fever had a throat smear stained with Gram's method, revealing Gram-positive cocci in chains. What is the most likely causative agent?
- A. Streptococcus pyogenes
- B. Staphylococcus aureus
- C. Neisseria gonorrhoeae
- D. Enterococcus faecalis
Correct Answer: A
Rationale: The correct answer is A: Streptococcus pyogenes. This bacterium is known to cause sore throat and fever, commonly referred to as strep throat. Gram's stain showing Gram-positive cocci in chains is characteristic of Streptococcus species. Streptococcus pyogenes is a beta-hemolytic bacterium that forms chains and is a common cause of bacterial pharyngitis. Staphylococcus aureus (B) is a Gram-positive cocci that forms clusters, not chains. Neisseria gonorrhoeae (C) is a Gram-negative diplococcus and is not associated with sore throat. Enterococcus faecalis (D) is a Gram-positive cocci that can cause infections but is not typically associated with sore throat.
Which description matches most to adenoviruses:
- A. Middle sized, naked icosahedral capsid, double-stranded DNA
- B. Small-sized, enveloped capsid, ssRNA
- C. Large, enveloped capsid, double stranded DNA
- D. None of the above
Correct Answer: A
Rationale: Step-by-step rationale:
1. Adenoviruses are DNA viruses, so choices B and C (ssRNA and DNA) are incorrect.
2. Adenoviruses have a middle-sized, naked icosahedral capsid, making choice A the correct description.
3. Choice D is incorrect as adenoviruses do have specific characteristics, making "None of the above" an inaccurate option.
In summary, the correct answer A is supported by the fact that adenoviruses have a middle-sized, naked icosahedral capsid and contain double-stranded DNA. Choices B, C, and D are incorrect as they do not align with the characteristics of adenoviruses.