Practice MDCAT Biology Hard MCQs with explanations — PMC, ETEA, NUMS entry test preparation.
Q1. A patient is diagnosed with an RNA virus that uses an enzyme to synthesize a DNA template. Which enzyme is essential for this specific viral replication?
Answer: Reverse transcriptase
Explanation: Reverse transcriptase converts RNA to DNA in retroviruses. RNA polymerase synthesizes RNA from DNA, which is the opposite of the required process.
Q2. A blood transfusion recipient develops jaundice. Laboratory tests confirm a DNA virus that is often referred to as serum hepatitis. Which virus is this?
Answer: Hepatitis B
Explanation: Hepatitis B is a DNA virus known as serum hepatitis. Hepatitis A is an RNA virus typically transmitted via the feco-oral route.
Q3. An HIV particle approaches a human T-lymphocyte. Which specific surface glycoprotein on the virus binds to the CD4 receptor of the host cell?
Answer: gp120
Explanation: gp120 is the surface spike that binds CD4. gp41 is involved in fusion, and p24 is a core protein, not a surface receptor-binder.
Q4. Under stressful conditions, a lysogenic bacterial population suddenly begins to lyse. Which process triggers the transition from the lysogenic to the lytic cycle?
Answer: Induction
Explanation: Induction is the process where a prophage detaches to start the lytic cycle. Transduction involves horizontal gene transfer, not the cycle switch.
Q5. A patient recovers from a viral infection but remains a chronic carrier of a virus previously known as 'non-A, non-B hepatitis.' Which virus is this?
Answer: Hepatitis C
Explanation: Hepatitis C was historically called non-A, non-B hepatitis. Hepatitis E is also non-A non-B but is typically acute and feco-oral.
Q6. A patient exhibits rapid cognitive decline and brain vacuolation. The infectious agent lacks nucleic acids. What is the most likely cause?
Answer: Prions
Explanation: Prions are purely proteinaceous and cause neurodegenerative diseases; viroids fail because they consist only of naked circular RNA molecules.
Q7. A 30-year-old man experiences recurring cold sores. Which characteristic of the causative virus allows it to persist in his nerve cells?
Answer: Herpes simplex
Explanation: Herpes simplex virus remains latent in the nerve ganglia for years; HIV fails because it integrates as a provirus in lymphocytes.
Q8. A plant pathologist identifies an infectious agent consisting only of a short strand of circular RNA. This agent is classified as a:
Answer: Viroids
Explanation: Viroids are composed only of circular RNA without a protein coat; prions fail because they are composed only of proteins.
Q9. To establish a permanent infection, HIV must insert its genome into the host's DNA. Which enzyme facilitates this specific step?
Answer: Integrase
Explanation: Integrase is required to insert viral DNA into the host genome; Reverse transcriptase fails because it only synthesizes the DNA.
Q10. While most DNA viruses contain double-stranded genomes, a specific group contains single-stranded DNA. Which virus belongs to this exceptional group?
Answer: Parvovirus
Explanation: Parvoviruses are rare exceptions containing single-stranded DNA. Poxviruses are tempting because they are DNA viruses, but they are double-stranded.
Q11. Most RNA viruses contain single-stranded RNA; however, a specific exception exists. Which of these contains a double-stranded RNA genome?
Answer: Reovirus
Explanation: The Reovirus group contains double-stranded RNA, an exception to the ssRNA rule. Influenza is tempting but it contains single-stranded RNA.
Q12. A patient with chronic Hepatitis B is suddenly co-infected with another virus that cannot replicate on its own. What is this delta-agent?
Answer: Hepatitis D
Explanation: Hepatitis D is a defective virus that only replicates in the presence of Hepatitis B. Hepatitis E is tempting but it is independent.
Q13. The Central Dogma states information flows from DNA to RNA. Which group of viruses provides a significant exception by reversing this flow?
Answer: Retroviruses
Explanation: Retroviruses reverse the flow of genetic information from RNA to DNA. Bacteriophages are tempting but they follow the standard DNA-to-RNA flow.
Q14. In the study of unconventional pathogens like those causing Mad Cow Disease, researchers found the infectious agent lacks nucleic acids and consists of:
Answer: Proteins only
Explanation: Prions are purely proteinaceous infectious particles. Viroids consist only of circular RNA and are the most tempting wrong option for sub-viral agents.
Q15. Genetic reassortment occurs frequently in certain viruses due to their segmented RNA genomes. Which virus exhibits this characteristic according to PTB?
Answer: Influenza virus
Explanation: Influenza has a segmented genome allowing rapid mutation. HIV has two identical strands but is not considered segmented like the flu virus.
Q16. When HIV DNA is integrated into the human host's chromosome, it can remain latent for years. This integrated state is known as:
Answer: Provirus
Explanation: Provirus refers specifically to the integrated viral DNA in animal cells like HIV. Prophage is the equivalent term used for bacteriophages.
Q17. When comparing Viroids and Prions, which of the following structural features is common to both infectious agents?
Answer: Both lack a capsid
Explanation: Viroids are plant pathogens made of RNA. Prions are animal pathogens made of protein. Neither contains both protein and nucleic acid.
Q18. Within an HIV-infected cell, the newly synthesized viral DNA must enter the nucleus. Which enzyme facilitates its incorporation into the host's chromosomal DNA?
Answer: Integrase
Explanation: Integrase inserts viral DNA into the host genome. Protease is involved in the final maturation of viral proteins after they are translated, not integration.
Q19. Hepatitis C often goes undetected for years because it avoids immediate immune destruction. What is a primary mechanism for its high rate of chronic infection?
Answer: Frequent mutation of surface antigens
Explanation: Hepatitis C has a high mutation rate, leading to antigenic variation that evades the immune system. It does not integrate into host DNA like HIV.
Q20. The influenza virus frequently causes seasonal epidemics due to minor, gradual mutations in its surface proteins. What is this specific mechanism called?
Answer: Antigenic drift
Explanation: Antigenic drift refers to minor seasonal mutations. Antigenic shift involves major changes, often through reassortment, leading to pandemics rather than typical seasonal epidemics.
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