A patient presents with severe respiratory infection. Gram-staining reveals gram-negative rods that are oxidase-positive and motile. Which organism is most likely responsible?
Answer: A
Pseudomonas aeruginosa is a gram-negative, oxidase-positive rod commonly causing respiratory infections in hospitalized patients. E. coli and Klebsiella are oxidase-negative, while Proteus is also oxidase-negative.
Q.22Easy
Which of the following bacterial structures is responsible for horizontal gene transfer through direct cell-to-cell contact?
Answer: A
Pili (sex pili) mediate bacterial conjugation, enabling direct DNA transfer between cells. Flagella aid motility, capsules provide protection, and teichoic acids are cell wall components.
Q.23Medium
A laboratory identifies a virus with positive-sense, single-stranded RNA genome. Which process can this virus directly initiate without viral RNA-dependent RNA polymerase?
Answer: A
Positive-sense RNA can directly serve as mRNA for ribosomal translation. Replication requires RNA-dependent RNA polymerase. This genome type doesn't undergo reverse transcription or DNA synthesis.
Q.24Easy
Which bacterial genus produces endospores as a survival mechanism under nutrient-depleted conditions?
Answer: A
Bacillus and Clostridium are the primary gram-positive bacteria capable of forming endospores. These structures are highly resistant to heat, chemicals, and radiation.
Q.25Medium
A researcher observes that a bacterial strain previously sensitive to penicillin now survives treatment. Analysis shows increased production of beta-lactamase. This exemplifies which mechanism of antibiotic resistance?
Answer: B
Beta-lactamase enzymatically cleaves the beta-lactam ring of penicillin, inactivating the antibiotic. This is enzymatic degradation, not target modification or pump-mediated resistance.
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Q.26Medium
In a clinical sample, Mycobacterium tuberculosis appears as acid-fast positive. This property is due to which cell wall component?
Answer: B
Mycolic acids in the mycobacterial cell wall resist acid-alcohol decolorization, causing the characteristic acid-fast staining. These long-chain fatty acids are unique to mycobacteria.
Q.27Easy
Which virus possesses a segmented genome and is responsible for causing influenza pandemics?
Answer: A
Influenza A virus has 8 RNA segments, enabling antigenic reassortment. This segmentation allows rapid evolution and pandemic potential through genetic exchange with other influenza strains.
Q.28Hard
A viral infection shows symptoms of hemorrhagic fever with high mortality. Laboratory tests reveal negative-sense RNA and a helical capsid. Which family of viruses is most likely?
Answer: B
Filoviruses (Ebola, Marburg) cause hemorrhagic fevers, possess negative-sense RNA, and display characteristic helical/filamentous morphology with extreme pathogenicity.
Q.29Medium
Which bacterial flagellar protein is the primary target of monoclonal antibodies used in serological identification?
Answer: A
Flagellin comprises the helical filament and constitutes the H (Hauch) antigen, used for serotyping bacteria like Salmonella. Other flagellar components are internal and less immunogenic.
Q.30Easy
A gram-positive coccus forms grape-like clusters and produces alpha-toxin. Which pathogen is being described?
Answer: B
Staphylococcus aureus characteristically forms clusters (not chains), produces alpha-hemolysin (alpha-toxin), and is coagulase-positive, distinguishing it from streptococci.
Q.31Medium
In viral replication, which process involves the insertion of viral DNA into host chromosomes, as seen in retroviruses?
Answer: B
Integration is the process by which retroviral DNA (synthesized via reverse transcriptase) becomes incorporated into host genomic DNA. This establishes persistent infection and can cause insertional mutagenesis.
Q.32Hard
A bacterium isolated from contaminated water shows resistance to multiple antibiotics and possesses a plasmid. Which method would effectively identify if the plasmid carries resistance genes?
Answer: A
PCR with specific primers targeting antibiotic resistance genes (like mecA, blaTEM) directly amplifies and identifies plasmid-encoded resistance. Other methods lack this molecular specificity.
Q.33Easy
Which viral protein is responsible for recognizing and binding to host cell receptors during viral attachment?
Answer: B
Surface glycoproteins (spike proteins in coronaviruses, gp120 in HIV) mediate host cell recognition and attachment. Matrix proteins provide structural support; enzymes have catalytic roles.
Q.34Medium
A patient with chronic Pseudomonas aeruginosa lung infection shows biofilm formation. Which virulence factor primarily enables this?
Answer: B
Pseudomonas aeruginosa produces alginate, a viscous polysaccharide exopolymer that forms the biofilm matrix, protecting bacteria from antibiotics and immune responses. This is characteristic of chronic respiratory infections.
Q.35Hard
Which of the following accurately describes the replication cycle of a negative-sense RNA virus?
Answer: B
Negative-sense RNA viruses must first synthesize complementary positive-sense RNA using viral RNA-dependent RNA polymerase. This positive-sense RNA serves as mRNA and template for new negative-sense genomes.
Q.36Easy
A bacterial colony on blood agar shows beta-hemolysis and is catalase-positive, coagulase-positive. Which species is confirmed?
Answer: C
Staphylococcus aureus is catalase-positive (distinguishes from streptococci) and coagulase-positive (specific identification). Beta-hemolysis results from alpha-toxin production.
Q.37Medium
In SARS-CoV-2, which structural protein is responsible for membrane fusion and is the primary target of neutralizing antibodies?
Answer: B
The spike protein mediates receptor binding and membrane fusion. It's the most exposed surface protein and elicits the strongest neutralizing antibody response, making it the vaccine target.
Q.38Medium
Which bacterial genus requires X factor (hemin) and V factor (NAD) for growth, making it fastidious?
Answer: B
Haemophilus species are fastidious gram-negative coccobacilli requiring both X factor (hemin) and V factor (NAD). Culture on chocolate agar provides these factors; they won't grow on regular blood agar.
Q.39Hard
A virus exhibits tropism exclusively for liver hepatocytes. Which viral component determines this tissue-specific infectivity?
Answer: C
Viral tropism is determined by complementary interactions between viral attachment proteins and specific cell surface receptors. Hepatitis B, C viruses demonstrate hepatocyte tropism through receptor-specific binding.
Q.40Hard
In 2024, which antimicrobial stewardship approach has been emphasized to combat carbapenem-resistant Enterobacteriaceae (CRE)?
Answer: B
Current stewardship guidelines recommend combination therapy (often with colistin or fosfomycin), strict infection control, and judicious carbapenem use to prevent further resistance escalation in CRE.
