Mycobacterial and Fungal Diagnostics (NAAT, LPA, Culture, DST, IGRA, Galactomannan) MCQs

Microbiology · 44 free questions with answers & explanations.

  1. An IGRA (QuantiFERON-TB Gold Plus) result shows: TB1 (CD4 peptides) = 1.2 IU/mL, TB2 (CD4+CD8 peptides) = 1.8 IU/mL, Nil = 0.1 IU/mL, Mitogen = 8.5 IU/mL. This IGRA result is interpreted as:
  2. A neutropenic patient post-allogenic stem cell transplant develops fever unresponsive to broad-spectrum antibiotics. CT thorax shows multiple nodules with a 'halo sign.' Serum galactomannan (ELISA) is ordered. The galactomannan assay detects a component of the cell wall of:
  3. Line probe assay (GenoType MTBDRsl) is used to detect second-line drug resistance in MTB. This assay detects mutations conferring resistance to fluoroquinolones and second-line injectables. Which gene mutations are detected for fluoroquinolone resistance?
  4. A patient with suspected pulmonary TB in India is evaluated for INH and rifampicin resistance. GeneXpert MTB/RIF Ultra is performed on sputum. The test detects rifampicin resistance by:
  5. IGRA (QuantiFERON-TB Gold Plus) is positive in a 30-year-old healthcare worker. TST (Mantoux) is also 18 mm. The worker has no symptoms, normal CXR, and no prior BCG record. How should these results be interpreted?
  6. Serum beta-D-glucan is 800 pg/mL (reference <80) in a febrile neutropenic patient. Galactomannan (GM) index is 0.3 (negative). Cryptococcal antigen is negative. Which infections are most likely?
  7. A 48-year-old post-renal transplant patient on tacrolimus + prednisolone develops fever and pulmonary infiltrates. Serum galactomannan (GM) ELISA index is 1.8 (cut-off >0.5). Beta-D-glucan is also elevated. CT thorax shows a halo sign. Which antifungal should be started, and what is the limitation of the galactomannan test?
  8. A healthcare worker with a previous history of BCG vaccination has a QuantiFERON-TB Gold In-Tube (QFT-GIT) result of 0.38 IU/mL (reactive/positive; cut-off ≥0.35). TST (Mantoux) is 12 mm. What is the preferred interpretation and clinical decision?
  9. The Xpert MTB/RIF Ultra assay (GeneXpert Ultra) is performed on a sputum specimen from a suspected pulmonary TB patient with HIV. The result shows 'MTB detected TRACE; RIF resistance NOT detected'. What is the correct interpretation?
  10. A Line Probe Assay (Hain LPA GenoType MTBDRplus) is performed on a sputum smear-positive specimen. The probe hybridisation pattern shows: absence of the WT2 band (rpoB region) and presence of the MUT3A band (rpoB S531L), plus absence of WT1 band (katG region) and presence of MUT1 band (katG S315T1). What drug resistance pattern is indicated?
  11. A neutropenic bone marrow transplant patient has serial serum Galactomannan (GM) testing. On day 14 post-transplant, the GM index rises from 0.3 to 1.6 on ELISA (cut-off ≥0.5 for single test in BMT; ≥0.7 for serial monitoring). CT chest shows a new nodule. GM is a cell wall component released during active growth of which fungal genus?
  12. A neutropenic patient with haematological malignancy develops fever unresponsive to broad-spectrum antibiotics. Serum galactomannan (GM) index is 1.8 (positive cutoff ≥0.5) on two consecutive samples. Chest CT shows a new nodule with a 'halo sign'. The most likely diagnosis and first-line treatment are:
  13. QuantiFERON-TB Gold Plus (QFT-Plus) IGRA measures the release of IFN-γ from CD4 T-cells stimulated by TB-specific antigens. Which antigens are used, and why are they preferred over tuberculin (PPD) for LTBI screening?
  14. The Xpert MTB/RIF assay (GeneXpert) simultaneously detects M. tuberculosis complex and rifampicin resistance in under 2 hours. Rifampicin resistance is detected by identifying mutations in which region?
  15. A 45-year-old immunocompetent patient from Rajasthan presents with a chronic pulmonary nodule, skin ulcer over the hand, and draining sinus discharging black grains. Fungal culture of the grains on Sabouraud's medium grows dark, dematiaceous hyphae. Direct KOH shows characteristic 'copper penny' sclerotic bodies (muriform cells). The most likely diagnosis is:
  16. A healthcare worker has a history of BCG vaccination at birth. An IGRA (QuantiFERON-TB Gold Plus) test shows IFN-γ release of 0.8 IU/mL in TB1 (ESAT-6/CFP-10) and 0.3 IU/mL in the Nil control. The result is interpreted as POSITIVE. Which statement best explains why IGRA is preferred over TST in BCG-vaccinated individuals?
  17. Beta-D-glucan (BDG) assay is used as a pan-fungal biomarker. Which organisms produce a FALSE-NEGATIVE BDG result despite causing invasive fungal infection?
  18. When performing drug susceptibility testing (DST) for Mycobacterium tuberculosis by the proportion method on Lowenstein-Jensen medium, resistance is defined as growth on drug-containing medium that exceeds what percentage of the control?
  19. A 40-year-old man presents with productive cough for 3 months. Two sputum smears are AFB negative. Xpert MTB/RIF Ultra is performed and reports MTB detected (very low) with rifampicin indeterminate. What is the clinical interpretation and next step?
  20. An immunocompromised patient on prolonged high-dose corticosteroids develops fever unresponsive to antibiotics and a new pulmonary infiltrate. Bronchoalveolar lavage (BAL) galactomannan index is 2.8. What is the most likely diagnosis and the recommended first-line treatment?
  21. The QuantiFERON-TB Gold Plus (QFT-Plus) IGRA measures IFN-gamma release in response to two TB-specific peptide antigens. Which antigens are used, and why is BCG vaccination not a cause of false-positive IGRA results?
  22. A urine Lateral Flow Assay (LFA) for Cryptococcal antigen (CrAg) in a newly diagnosed HIV patient (CD4 8 cells/µL) is reactive at 1:160 dilution. The patient is asymptomatic with no meningism. What is the WHO recommended management?
  23. A febrile neutropenic patient 2 weeks post allogeneic stem cell transplant has progressive bilateral nodular infiltrates on HRCT chest. Serum galactomannan (GM) ELISA index is 0.65 (cut-off 0.5). Repeat serum GM after 2 days is 1.2. What is the most appropriate next step for diagnosis confirmation?
  24. An IGRA (QuantiFERON-TB Gold Plus, QFT-Plus) test returns an indeterminate result in a healthcare worker undergoing pre-employment TB screening. QFT-Plus uses two antigen tubes (TB1 with ESAT-6/CFP-10 CD4 stimulation, TB2 with CD8 as well). Which of the following scenarios most commonly causes an indeterminate QFT-Plus result?
  25. Cryptococcus neoformans meningitis is confirmed by India ink preparation of CSF showing budding yeast with a wide polysaccharide capsule. Serum cryptococcal antigen (CrAg) lateral flow assay (LFA) returns positive at titre 1:512. What is the most important prognostic marker in this infection?
  26. Histoplasma capsulatum var. capsulatum is isolated from BAL of an immunocompromised patient from Madhya Pradesh with persistent fever and hepatosplenomegaly. On Sabouraud's agar at 25°C, the colony produces microconidia and distinctive tuberculate macroconidia. What is the recommended treatment for severe disseminated histoplasmosis?
  27. A 38-year-old male with HIV (CD4 count 60 cells/µL) has fever, cough, and bilateral pulmonary infiltrates. Bronchoalveolar lavage shows beta-1,3-D-glucan 380 pg/mL (positive) and galactomannan ODI of 3.2. The most likely diagnosis is:
  28. Line Probe Assay (LPA) for rifampicin resistance in tuberculosis targets mutations primarily in which region of the rpoB gene?
  29. An IGRA (interferon-gamma release assay) for tuberculosis infection uses antigens ESAT-6 and CFP-10, which are encoded within the RD1 region. These antigens are absent in BCG strains because:
  30. The Xpert MTB/RIF Ultra assay has improved sensitivity over the standard Xpert MTB/RIF particularly for which patient population?
  31. Which culture medium is most appropriate for primary isolation of Histoplasma capsulatum from a sputum specimen of an immunocompromised patient?
  32. In phenotypic drug susceptibility testing (DST) for M. tuberculosis on Lowenstein-Jensen medium, isoniazid resistance is tested at two concentrations: 0.2 µg/mL and 1.0 µg/mL. Growth at 0.2 µg/mL but not 1.0 µg/mL indicates:
  33. A 34-year-old HIV-positive patient (CD4 count 60 cells/µL) with fever, productive cough, and night sweats has a sputum AFB smear that is 2+ positive. Xpert MTB/RIF assay detects MTB and reports rifampicin resistance (rpoB mutation). What should be the IMMEDIATE next step?
  34. An QuantiFERON-TB Gold Plus (QFT-Plus) assay is performed on a healthcare worker for latent TB screening. The result shows: TB1 tube IFN-γ = 0.60 IU/mL, TB2 tube IFN-γ = 0.55 IU/mL, Nil = 0.20 IU/mL, Mitogen = 8.0 IU/mL. How should this result be interpreted?
  35. A neutropenic patient post-allogeneic bone marrow transplant develops persistent fever despite broad-spectrum antibiotics. HRCT chest shows a nodule with a halo sign. Serum galactomannan (Platelia Aspergillus ELISA) optical density index is 2.4. Which of the following is TRUE regarding galactomannan testing?
  36. Line probe assay (GenoType MTBDRplus) for first-line drug susceptibility testing detects mutations in which gene pair to identify isoniazid resistance?
  37. A 55-year-old renal transplant patient on tacrolimus develops subacute meningitis. CSF India ink preparation shows encapsulated yeast cells. CSF cryptococcal antigen (CrAg) lateral flow assay is strongly positive (titer 1:2048). The serum CrAg LFA is also positive. Which of the following about the CrAg LFA is CORRECT?
  38. Mycobacterium tuberculosis is cultured from a pulmonary sample on Lowenstein-Jensen (LJ) medium. The colonies appear after 4 weeks and show a characteristic morphology. Which of the following best describes M. tuberculosis colonies on LJ medium?
  39. A patient with pulmonary TB has sputum smear positive for AFB but culture on Löwenstein-Jensen (LJ) medium is negative after 8 weeks of incubation. The most likely explanation is:
  40. Line probe assay (LPA — Hain GenoType MTBDRplus) detects resistance to rifampicin by identifying mutations in which gene?
  41. An immunocompromised patient has a serum galactomannan index of 1.8 (positive cut-off ≥0.5) on two consecutive samples. CT chest shows a halo sign. BAL galactomannan is also elevated. The most appropriate initial antifungal treatment is:
  42. QuantiFERON-TB Gold Plus (QFT-Plus) differs from the older QuantiFERON-TB Gold In-Tube (QFT-GIT) in that QFT-Plus additionally measures:
  43. A patient with AIDS (CD4 count 45 cells/µL) presents with fever, headache, and meningism. CSF India ink preparation reveals encapsulated budding yeasts. Serum cryptococcal antigen (CrAg) is 1:1024. First-line induction treatment in this resource-limited setting is:
  44. Xpert MTB/RIF Ultra assay has higher sensitivity than the standard Xpert MTB/RIF for paucibacillary tuberculosis primarily because of:
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