Practice MDCAT Biology Medium MCQs with explanations — PMC, ETEA, NUMS entry test preparation.
Q1. A clinician observes a virus that acquires its outer lipid bilayer from the host cell's nuclear membrane. This characteristic is diagnostic of which viral group?
Answer: Herpes virus
Explanation: Herpes virus is enveloped, acquiring its lipid layer from the host nuclear membrane. Bacteriophages are typically naked, lacking such envelopes.
Q2. In a laboratory culture of E. coli, a viral genome integrates into the bacterial chromosome and replicates along with it. What is this integrated viral DNA termed?
Answer: Prophage
Explanation: The prophage is the integrated viral genome in lysogeny. A provirus refers specifically to integrated eukaryotic viral DNA like HIV.
Q3. During the attachment phase of a T4 bacteriophage to an E. coli cell, which specific structure facilitates the initial recognition of the receptor sites?
Answer: Tail fibers
Explanation: Tail fibers are responsible for attachment to receptor sites. The tail sheath contracts to inject DNA, but it does not perform recognition.
Q4. A patient develops painful vascular lesions in the mouth and lips. Which specific virus is the most common cause of these oral lesions?
Answer: Herpes simplex type 1
Explanation: Herpes simplex type 1 causes oral lesions. Type 2 is primarily responsible for genital herpes, though both belong to the same family.
Q5. A plant pathologist identifies a small, circular, naked RNA molecule causing stunted growth in potatoes. What is this pathogen classified as?
Answer: Viroid
Explanation: Viroids are naked RNA molecules. Prions are proteins, and virions are complete viral particles containing both nucleic acid and protein.
Q6. An elderly man develops a respiratory infection characterized by frequent changes in viral surface antigens. Which virus is known for this high mutation rate?
Answer: Influenza virus
Explanation: Influenza viruses undergo frequent antigenic changes. Rhinovirus causes the common cold but is not as famous for major antigenic shifts like Influenza.
Q7. During the penetration stage of a T4 bacteriophage infection, which enzyme is released to weaken the bacterial cell wall?
Answer: Lysozyme
Explanation: Lysozyme digests the bacterial peptidoglycan wall. Penicillin is an antibiotic that inhibits wall synthesis but is not produced by viruses.
Q8. A patient diagnosed with HIV shows high viral replication. Which enzyme is responsible for converting the single-stranded viral RNA into double-stranded DNA?
Answer: Reverse transcriptase
Explanation: Reverse transcriptase synthesizes DNA from an RNA template; RNA polymerase fails because it produces RNA from a DNA template.
Q9. A medical student examines a virus containing double-stranded DNA and an envelope, transmitted through blood. Which virus is being studied?
Answer: Hepatitis B
Explanation: Hepatitis B is the only DNA virus among the common hepatitis types; Hepatitis A is a tempting wrong choice but is RNA.
Q10. During HIV entry into a T-helper cell, which viral component is primarily responsible for the initial adsorption to the CD4 receptor?
Answer: gp120
Explanation: The gp120 spike specifically binds to the CD4 receptor; gp41 fails because it mediates the fusion of the viral envelope.
Q11. A patient presents with high fever and respiratory distress caused by an enveloped RNA virus. Which of the following fits this description?
Answer: Influenza virus
Explanation: Influenza is an enveloped RNA virus; Poliovirus is a tempting wrong choice because it is a naked (non-enveloped) RNA virus.
Q12. A patient develops chronic liver disease after a blood transfusion. The causative agent is Hepatitis C. What is its genomic nature?
Answer: RNA virus
Explanation: Hepatitis C is an enveloped RNA virus often called non-A non-B; Hepatitis B fails as it is a DNA virus.
Q13. In the HIV lifecycle, the viral DNA is integrated into the host cell's chromosome. What is the virus called in this state?
Answer: Provirus
Explanation: A provirus is the integrated viral DNA in a eukaryotic host; a prophage specifically refers to integration in a bacterial host.
Q14. A patient recovers quickly from 'infectious hepatitis' after eating contaminated shellfish. Which virus, characterized by a short incubation, was likely responsible?
Answer: Hepatitis A
Explanation: Hepatitis A has a short incubation period and is transmitted fecally; Hepatitis B fails because it has a long incubation period.
Q15. A virologist classifies a new virus based on its ability to only infect specific species of birds. This classification system uses:
Answer: Host range
Explanation: Viruses are primarily classified based on whether they infect plants, animals, or bacteria; symmetry fails as it is a secondary structural feature.
Q16. During a laboratory analysis of viral replication, a researcher observes a DNA virus replicating entirely within the host's cytoplasm. Which virus is being studied?
Answer: Poxvirus
Explanation: Poxviruses are unique DNA viruses that replicate in the cytoplasm. Herpesviruses are tempting because they are also DNA viruses but replicate in the nucleus.
Q17. A plant pathologist discovers an infectious agent causing stunted growth in potatoes that lacks a capsid and consists only of RNA. What is this agent?
Answer: Viroids
Explanation: Viroids consist solely of circular RNA without a protein coat. Prions are tempting but they consist only of proteins without nucleic acids.
Q18. A bacteriophage infects E. coli but does not immediately lyse the cell, instead persisting through generations. Which life cycle is being exhibited?
Answer: Lysogenic cycle
Explanation: In lysogeny, the viral DNA integrates into the host chromosome as a prophage. The lytic cycle is tempting but involves immediate host cell destruction.
Q19. An HIV particle enters a T-lymphocyte and begins synthesizing DNA from its RNA template. Which viral enzyme is responsible for this specific anomaly?
Answer: Reverse Transcriptase
Explanation: Reverse transcriptase converts viral RNA into DNA. RNA polymerase is tempting but it typically synthesizes RNA from a DNA template in normal cells.
Q20. A virologist identifies an animal virus that is resistant to ether treatment due to the absence of a lipid envelope. Which virus is this?
Answer: Picornavirus
Explanation: Picornaviruses are non-enveloped animal viruses. Myxoviruses are tempting because they also infect animals but they possess a lipid envelope.
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