Radioimmunoassay (RIA) MCQs With Answer

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

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