Radioimmunoassay (RIA) MCQs With Answer for M. Pharm Students
Radioimmunoassay (RIA) remains a cornerstone technique for quantifying minute concentrations of hormones, drugs, and biomarkers with exceptional sensitivity. This quiz-oriented blog is tailored for M. Pharm students to reinforce core concepts: competitive binding principles, tracer selection, separation strategies, curve fitting, quality control, and radiation safety. You will encounter application-focused questions covering I-125 gamma counting, %B/B0 calculations, non-specific binding, cross-reactivity, and advanced formats like IRMA. Each MCQ is followed by the correct answer to support immediate self-evaluation. Use these questions to deepen your understanding of assay design, optimization, and validation, and to bridge theoretical knowledge with practical decision-making in Modern Pharmaceutical Analytical Techniques.
Q1. Which statement best describes the principle of a classical competitive RIA?
- Radiolabeled antibody binds two epitopes on the antigen to form a sandwich
- Radiolabeled antigen competes with unlabeled antigen for a limited number of antibody sites
- Unlabeled antibody competes with labeled antibody for antigen epitopes
- Antigen catalyzes conversion of a radiolabeled substrate to a product
Correct Answer: Radiolabeled antigen competes with unlabeled antigen for a limited number of antibody sites
Q2. The most commonly used radioisotope for labeling peptide hormones in RIA is:
- Iodine-125 (I-125)
- Phosphorus-32 (P-32)
- Carbon-14 (C-14)
- Iodine-131 (I-131)
Correct Answer: Iodine-125 (I-125)
Q3. In a classical competitive RIA, the radiolabeled component is typically the:
- Primary antibody
- Secondary antibody
- Antigen (tracer)
- Blocking protein (e.g., BSA)
Correct Answer: Antigen (tracer)
Q4. Which separation method is commonly used to distinguish bound from free antigen in RIA?
- Dextran-coated charcoal adsorption
- Gas chromatography
- Capillary electrophoresis
- Reversed-phase HPLC
Correct Answer: Dextran-coated charcoal adsorption
Q5. When logit(%B/B0) is plotted against log concentration of analyte in a competitive RIA, the relationship is generally:
- Quadratic
- Linear
- Exponential
- Hyperbolic
Correct Answer: Linear
Q6. The typical sensitivity (limit of detection) of a well-optimized RIA is in the:
- Millimolar range
- Micromolar range
- Picomolar to femtomolar range
- Nanomolar to micromolar range
Correct Answer: Picomolar to femtomolar range
Q7. For assays using I-125, radioactivity is most appropriately measured with a:
- Gamma counter with NaI(Tl) well detector
- Liquid scintillation counter with toluene-based cocktail
- UV-Vis spectrophotometer at 280 nm
- Fluorescence spectrophotometer
Correct Answer: Gamma counter with NaI(Tl) well detector
Q8. The physical half-life of Iodine-125 is approximately:
- 8 days
- 60 days
- 14 hours
- 12.3 years
Correct Answer: 60 days
Q9. In a competitive RIA, as the concentration of unlabeled analyte in the sample increases, the radioactivity measured in the bound fraction will:
- Increase
- Decrease
- Remain constant
- First increase then decrease
Correct Answer: Decrease
Q10. In RIA, “NSB” refers to:
- Non-specific binding of tracer to assay tubes and serum proteins independent of antibody
- Non-saturable binding of antigen to antibody
- Non-standard blank prepared without tracer
- Non-specific beta radiation from I-125
Correct Answer: Non-specific binding of tracer to assay tubes and serum proteins independent of antibody
Q11. Which statement correctly distinguishes IRMA (immunoradiometric assay) from classical RIA?
- IRMA is competitive and uses radiolabeled antigen
- IRMA is non-competitive and uses radiolabeled antibody
- IRMA does not use antibodies
- IRMA detects only large proteins and not small molecules
Correct Answer: IRMA is non-competitive and uses radiolabeled antibody
Q12. A key approach to detect matrix effects and validate assay specificity in RIA is to perform:
- Single-point calibration at the IC50 concentration
- Serial dilution of the sample to assess parallelism with the standard curve
- Washing the tubes with distilled water only
- Counting each tube for half the usual time
Correct Answer: Serial dilution of the sample to assess parallelism with the standard curve
Q13. In the dextran-coated charcoal method, the charcoal primarily removes:
- Antibody-bound radiolabeled antigen
- Free radiolabeled antigen
- Antibody aggregates only
- All radioactivity indiscriminately
Correct Answer: Free radiolabeled antigen
Q14. Appropriate shielding for safe handling of I-125 in the laboratory is:
- Plexiglass (acrylic) sheets
- Lead shielding with tongs to maximize distance
- Aluminum foil wrap
- No shielding is required
Correct Answer: Lead shielding with tongs to maximize distance
Q15. Carrier proteins such as BSA are added to RIA buffers primarily to:
- Increase gamma photon energy
- Promote tracer adsorption to tube walls
- Reduce non-specific adsorption and stabilize antibody/tracer
- Accelerate radioactive decay
Correct Answer: Reduce non-specific adsorption and stabilize antibody/tracer
Q16. In competitive RIA, B0 (B-zero) is best defined as:
- Counts bound when no tracer is added
- Counts bound in the presence of saturating unlabeled antigen
- Counts bound in the zero-analyte standard (maximum specific binding)
- Total counts added (TCA)
Correct Answer: Counts bound in the zero-analyte standard (maximum specific binding)
Q17. Significant antibody cross-reactivity with a drug’s metabolite in RIA typically causes:
- Underestimation of the analyte concentration
- Overestimation (positive bias) of the analyte concentration
- No effect on the measured concentration
- Randomly alternating over- and underestimation
Correct Answer: Overestimation (positive bias) of the analyte concentration
Q18. The standard curve in a competitive RIA is typically plotted as:
- %B/B0 versus log concentration of analyte
- Raw CPM versus linear concentration
- CPM of free fraction versus time
- Absorbance versus concentration
Correct Answer: %B/B0 versus log concentration of analyte
Q19. In the double-antibody separation method, the second antibody is directed against:
- The antigen’s epitope
- The primary antibody’s species IgG (anti-IgG)
- The radiolabel (iodine)
- Dextran-coated charcoal
Correct Answer: The primary antibody’s species IgG (anti-IgG)
Q20. In competitive RIA, the IC50 corresponds to the analyte concentration that yields approximately:
- 10% of B0
- 25% of B0
- 50% of B0
- 90% of B0
Correct Answer: 50% of B0
