Applied Microbiology and Serology MCQs

Microbiology · 41 free questions with answers & explanations.

1. A patient is tested for syphilis. VDRL test is positive at 1:8 dilution. To confirm the diagnosis, the specific treponemal test used is:
2. The Quellung (capsule swelling) reaction, used for serotyping encapsulated bacteria such as Streptococcus pneumoniae, is based on which principle?
3. In the complement fixation test (CFT) used for serodiagnosis, a NEGATIVE result (patient serum does NOT contain antibody to the antigen) is indicated by:
4. A patient has a positive ELISA for HIV but a negative Western blot with bands appearing at gp41 only. The MOST appropriate interpretation is:
5. ELISA for anti-HIV uses a 4th generation (combination) format. What makes 4th generation ELISA superior to 3rd generation for early HIV detection?
6. In the complement fixation test (CFT), a positive result (disease/antibody present) is indicated by:
7. In a multiplex PCR-based respiratory syndromic panel for a ventilated patient with hospital-acquired pneumonia, both Klebsiella pneumoniae and a CRKP-associated KPC resistance gene are detected. What is the most appropriate interpretation and immediate clinical action?
8. An ELISA test for HIV antibody detection has a sensitivity of 99.5% and specificity of 99.2%. In a screening programme in a population where HIV prevalence is 0.1%, what is the positive predictive value (PPV) of a positive ELISA result?
9. The ELISA technique used for quantitative measurement of serum cytokines uses which specific variant to achieve the highest sensitivity for detecting low concentrations?
10. Procalcitonin (PCT) is used as a biomarker to guide antibiotic stewardship in respiratory infections. What is the biological basis for elevated PCT in bacterial sepsis compared to viral respiratory infections?
11. In Western blot (immunoblot) confirmation of HIV-1 infection, which band patterns are required by WHO/CDC criteria for a confirmed positive result?
12. The complement fixation test (CFT) is used for serological diagnosis of several infections. In a positive CFT, what is the expected result when sheep red blood cells and hemolysin (sensitizing antibody) are added as an indicator system?
13. Polymerase chain reaction (PCR) is used widely for pathogen detection. Which modification enables quantification of pathogen load (e.g., viral load in HIV, HBV, HCV) and is the basis of quantitative PCR?
14. ELISA (Enzyme-Linked Immunosorbent Assay) is the workhorse of serodiagnosis. In a sandwich (capture) ELISA used to detect an antigen, arrange the layers correctly from solid phase to detection:
15. Antibiotic susceptibility testing by the disk diffusion method (Kirby-Bauer) uses Mueller-Hinton agar. Which two special supplements must be added to Mueller-Hinton agar when testing Streptococcus pneumoniae and Haemophilus influenzae respectively?
16. ELISA in its indirect format is used to detect antibodies in patient serum. What is the role of the secondary antibody conjugated to enzyme in indirect ELISA?
17. Phage typing is used to sub-type Staphylococcus aureus isolates for epidemiological investigation of hospital outbreaks. Group I phages lyse strains associated with which clinical syndrome that is NOT bullous impetigo?
18. The Quellung (Neufeld's) reaction is used to identify and serotype encapsulated bacteria. What is the correct principle of this reaction?
19. A research laboratory is developing a new ELISA for diagnosing acute leptospirosis. In a double-antibody sandwich ELISA (indirect ELISA format) to detect patient antibodies, the stages in order are:
20. Real-time PCR (RT-qPCR) is used for quantitative viral load testing in HIV-infected patients. The molecular mechanism that allows real-time quantification of PCR product without gel electrophoresis using TaqMan probes is:
21. In the Widal test for enteric fever, O antibodies and H antibodies have different clinical implications. Which interpretation is MOST CORRECT regarding Widal test results?
22. ELISA for HIV detection (4th generation) detects both p24 antigen and anti-HIV antibodies simultaneously. Why is this combination advantageous over 3rd generation (antibody-only) ELISA, and what is the residual window period for 4th generation assays?
23. A patient's serum is tested with the Widal test on day 10 of fever. The results are: TO 1:160, TH 1:320, BTO 1:40, BTH 1:40. How should these results be interpreted?
24. A clinical laboratory validates a new rapid diagnostic test (RDT) for malaria using 200 confirmed positive and 200 confirmed negative samples. The test correctly identifies 186 positives and 190 negatives. What are the sensitivity and specificity respectively?
25. Western blotting is the confirmatory test for HIV. In the standard interpretation, which combination of bands constitutes a reactive (positive) result per WHO/CDC criteria?
26. In ELISA, which type uses a labelled antibody directed against the analyte antibody (sandwich format) and is typically used to detect antibodies in patient serum for diagnosis of infectious disease?
27. A laboratory receives a serum sample for hepatitis B profile with the following results: HBsAg negative, anti-HBs positive, HBeAg negative, anti-HBe negative, anti-HBc IgG positive. This pattern is most consistent with:
28. The minimum inhibitory concentration (MIC) is defined as the lowest concentration of an antimicrobial agent that:
29. In ELISA for detecting anti-HCV antibodies, a patient sample is tested using a 4th generation assay (combined antigen-antibody ELISA). The sensitivity window for anti-HCV detection (compared to HCV RNA NAAT) is approximately:
30. In a complement fixation test (CFT), a positive test result (patient serum has specific antibodies) shows:
31. A patient positive for HIV on ELISA undergoes Western blot. Bands are present at gp120 and p24 only. According to WHO criteria, the Western blot result is:
32. Prozone phenomenon (false negative serological test result) is encountered in a case of secondary syphilis where VDRL appears negative at undiluted serum. After serial dilution, a reactive result is obtained at 1:16. What is the immunological mechanism of the prozone effect?
33. The Paul-Bunnell test detects heterophile antibodies in infectious mononucleosis. These antibodies agglutinate sheep RBCs but are absorbed by beef RBCs (not by Forssman antigen on guinea pig kidney cells). This pattern distinguishes heterophile antibodies from:
34. The prozone phenomenon in VDRL testing causes a false-negative result. The mechanism involves:
35. A blood culture bottle flags positive at 14 hours. Gram stain shows Gram-positive cocci in clusters. Rapid identification by MALDI-TOF MS identifies Staphylococcus aureus. The report also includes a Cefoxitin disc diffusion result showing an inhibition zone of 14 mm. How should this be interpreted?
36. A leptospirosis patient's serum is tested by the Microscopic Agglutination Test (MAT). Serum at dilution 1:800 shows 50% agglutination of Leptospira interrogans serovar Icterohaemorrhagiae live antigen. What does this titre signify, and what is the first-line treatment?
37. A 40-year-old cattle farmer presents with undulant fever, sweating, arthralgia and hepatosplenomegaly. Paired serology (acute and convalescent at 3 weeks) for Brucella shows a 4-fold rise in standard agglutination test (SAT) titre (1:80 to 1:320). Blood culture in BACTEC automated system grows a slowly growing, small, Gram-negative coccobacillus after 5 days of extended incubation. What is the treatment of choice for brucellosis?
38. ELISA (enzyme-linked immunosorbent assay) in the indirect format detects:
39. In immunochromatographic rapid diagnostic tests (RDTs) for malaria, the 'control line' appearing regardless of test outcome serves to:
40. In immunofluorescence (IF) techniques, the direct versus indirect method differ in the number of antibody layers used. Which statement correctly distinguishes direct from indirect immunofluorescence in diagnostic microbiology?
41. MALDI-TOF MS (matrix-assisted laser desorption ionisation — time of flight mass spectrometry) has revolutionised bacterial identification in clinical microbiology. Its primary principle of identification is: