A laboratory identifies a virus with single-stranded RNA genome that must first be transcribed to mRNA before protein synthesis. What is this virus classified as?
Answer: B
Negative-sense RNA viruses (-ssRNA) cannot be directly translated; they require RNA-dependent RNA polymerase to synthesize complementary positive-sense mRNA first.
Q.22Medium
In a viral outbreak investigation, electron microscopy reveals enveloped, pleomorphic particles (80-120 nm) with crown-like spike projections. Which family is most consistent?
Answer: C
Coronaviruses are characterized by crown-like spike proteins (S protein) on their enveloped surface, which are distinctive under electron microscopy.
Q.23Medium
A Gram-negative bacterium shows resistance to multiple antibiotics including cephalosporins. The presence of which enzymatic mechanism is most likely responsible?
Answer: B
ESBL-producing Gram-negative bacteria (E. coli, Klebsiella) hydrolyze third-generation cephalosporins, conferring resistance to multiple beta-lactam antibiotics.
Q.24Medium
A bacterium isolated from a urinary tract infection produces urease enzyme, enabling it to raise urine pH and form struvite crystals. Which organism is most likely?
Answer: B
Proteus mirabilis produces urease, alkalinizing urine and promoting precipitation of magnesium ammonium phosphate (struvite), forming characteristic staghorn calculi.
Q.25Medium
In viral replication cycles, retroviruses are unique in requiring which enzyme for reverse transcription of their RNA genome?
Answer: B
Reverse transcriptase converts retroviral ssRNA into dsDNA, a critical step unique to retroviruses before integration into the host chromosome via integrase.
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Q.26Medium
A patient with severe meningitis has CSF culture yielding Gram-positive cocci in chains with positive optochin test and bile solubility. What is the organism?
Answer: B
Streptococcus pneumoniae is distinguished by optochin sensitivity (alpha-hemolytic) and bile solubility, with lancet-shaped cocci causing bacterial meningitis.
Q.27Medium
Which bacteriophage classification system categorizes viruses based on nucleic acid type and presence/absence of envelopes?
Answer: A
Baltimore classification organizes viruses into 7 groups based on replication strategy and genetic material type, applicable to both bacterial and eukaryotic viruses.
Q.28Medium
A patient with diabetic foot ulcer yields Vibrio vulnificus from wound culture. Which environmental niche is this halophilic organism primarily associated with?
Answer: B
Vibrio vulnificus is a salt-tolerant marine bacterium causing wound infections through contaminated seawater exposure, particularly in immunocompromised individuals.
Q.29Medium
In bacterial conjugation, which plasmid type can transfer genes between different bacterial species through direct cell-to-cell contact?
Answer: A
R plasmids carrying antibiotic resistance genes are readily transferred via conjugation across species barriers, explaining rapid resistance dissemination in clinical settings.
Q.30Medium
Which of the following is a characteristic feature of SARS-CoV-2 that contributes to its high transmissibility?
Answer: A
The spike protein of SARS-CoV-2 has high affinity for ACE2 receptors, enabling efficient cellular entry and transmission.
Q.31Medium
A patient presents with atypical pneumonia. Which bacterial pathogen should be considered if Gram staining shows no organism but PCR is positive?
Answer: A
Mycoplasma lacks a cell wall and does not stain with Gram staining but is detectable by PCR, causing atypical pneumonia.
Q.32Medium
Which viral replication strategy involves reverse transcription of RNA to DNA?
Answer: A
Retroviruses like HIV use reverse transcriptase to synthesize DNA from their RNA genome for integration into host DNA.
Q.33Medium
Which bacterial structure mediates horizontal gene transfer through direct cell-to-cell contact?
Answer: A
The F pilus establishes a bridge between donor and recipient cells for plasmid transfer during bacterial conjugation.
Q.34Medium
A diagnostic lab suspects tuberculosis. Which staining technique is most appropriate for detecting Mycobacterium tuberculosis?
Answer: A
Mycobacterium tuberculosis has a waxy cell wall rich in mycolic acids that require acid-fast staining for visualization.
Q.35Medium
Which bacterial toxin directly inhibits protein synthesis by modifying EF-2 in host cells?
Answer: A
Diphtheria toxin catalyzes ADP-ribosylation of EF-2, blocking translation and causing severe systemic toxicity.
Q.36Medium
In viral latency, which herpesvirus establishes latency in sensory nerve ganglia?
Answer: A
HSV establishes latency in dorsal root ganglia and trigeminal ganglia, leading to recurrent herpes infections.
Q.37Medium
Which viral enzyme is responsible for integrating retroviral DNA into the host genome?
Answer: A
Integrase catalyzes the insertion of viral DNA into the host chromosome, establishing persistent infection in retroviruses like HIV.
Q.38Medium
A nosocomial outbreak of antibiotic-resistant Acinetobacter baumannii is investigated. Which transmission route is most likely?
Answer: A
A. baumannii survives on dry surfaces and spreads via contact; it is a major nosocomial pathogen in ICU settings.
Q.39Medium
A bacterial strain shows resistance to multiple antibiotics. Which genetic element is most likely responsible?
Answer: B
Plasmids are extra-chromosomal DNA elements that carry multiple antibiotic resistance genes and can be horizontally transferred between bacteria, leading to multi-drug resistance.
Q.40Medium
Which bacterial species is known to produce lipopolysaccharide (LPS) as an endotoxin?
Answer: B
E. coli is a gram-negative bacterium with LPS in its outer membrane, which acts as an endotoxin triggering inflammatory responses in host infections.
