Introduction: The use of radioactive isotopes in biogenetic studies provides powerful tools for tracing biochemical pathways, elucidating metabolic flux, and characterizing drug metabolism and pharmacokinetics. Radioisotopes such as 14C, 3H, 32P, 35S, 99mTc and 18F enable radiolabeling of small molecules, nucleic acids and proteins for autoradiography, scintillation counting, PET/SPECT imaging and receptor-binding assays. Understanding half-life, specific activity, radiochemical purity, detection methods and radiation safety is essential for B. Pharm students conducting tracer experiments or interpreting radiotracer data. This topic integrates theory and practical considerations including choice of isotope, labeling chemistry and data analysis. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which is the primary purpose of using radioactive isotopes in biogenetic studies?
- To increase chemical stability of compounds
- To act as catalysts in biochemical reactions
- To trace metabolic pathways and quantify flux
- To sterilize biological samples
Correct Answer: To trace metabolic pathways and quantify flux
Q2. Which radioisotope is preferred for long-term tracing of carbon skeletons in metabolic studies?
- 99mTc
- 14C
- 18F
- 131I
Correct Answer: 14C
Q3. Which isotope is commonly used to label DNA for high-sensitivity detection in molecular biology?
- 35S
- 32P
- 99mTc
- 18F
Correct Answer: 32P
Q4. For labelling exchangeable hydrogen positions in small molecules and lipids, which isotope is most suitable?
- 3H (tritium)
- 131I
- 99mTc
- 14C
Correct Answer: 3H (tritium)
Q5. What is the basic principle of autoradiography used in biogenetic studies?
- Radiation changes chemical structure producing color
- Radioactive decay generates heat detected by thermography
- Emitted radiation exposes a photographic emulsion or detector proportional to label distribution
- Radioisotopes fluoresce under UV light
Correct Answer: Emitted radiation exposes a photographic emulsion or detector proportional to label distribution
Q6. How is specific activity defined in radiotracer chemistry?
- Mass of compound per mole
- Radioactivity per unit mass or amount of compound
- Ratio of stable to radioactive isotope only
- Number of radioactive decays per minute in a detector
Correct Answer: Radioactivity per unit mass or amount of compound
Q7. A sample has an initial activity of 160 MBq and the isotope half-life is 5 days. What activity remains after 10 days?
- 80 MBq
- 40 MBq
- 20 MBq
- 120 MBq
Correct Answer: 40 MBq
Q8. Which SI unit represents one nuclear disintegration per second?
- Curie (Ci)
- Becquerel (Bq)
- Gray (Gy)
- Sievert (Sv)
Correct Answer: Becquerel (Bq)
Q9. What does radiochemical purity indicate for a labeled compound?
- Proportion of biological activity retained
- Fraction of total radioactivity present in the desired chemical form
- Specific activity expressed as Ci/mmol
- Total mass percent of radioactive material
Correct Answer: Fraction of total radioactivity present in the desired chemical form
Q10. Which imaging modality is typically used with gamma-emitting isotopes like 99mTc for organ imaging?
- PET (Positron Emission Tomography)
- SPECT (Single Photon Emission Computed Tomography)
- Ultrasound
- MRI (Magnetic Resonance Imaging)
Correct Answer: SPECT (Single Photon Emission Computed Tomography)
Q11. Which radioisotope is a common positron emitter used in PET for radiolabelled glucose analogs?
- 99mTc
- 18F
- 14C
- 131I
Correct Answer: 18F
Q12. Which isotope has an approximate physical half-life of 6 hours and is widely used in diagnostic nuclear medicine?
- 131I
- 99mTc
- 14C
- 32P
Correct Answer: 99mTc
Q13. Which detector is most appropriate for quantitative measurement of beta-emitting, liquid biological samples?
- Gamma camera
- Liquid scintillation counter
- Thermoluminescent dosimeter
- Optical spectrophotometer
Correct Answer: Liquid scintillation counter
Q14. What does the term “carrier-free” radioisotope mean?
- Isotope supplied without chemical solvent
- Radioisotope preparation containing no detectable non-radioactive (stable) isotope of the same element
- Isotope encapsulated in carrier protein
- Isotope bound to an inert carrier molecule
Correct Answer: Radioisotope preparation containing no detectable non-radioactive (stable) isotope of the same element
Q15. Which chemical method is commonly used for iodination of proteins and peptides in radiolabeling?
- Carbodiimide coupling
- Chloramine-T oxidation method
- EDC/NHS esterification
- Click chemistry with azides
Correct Answer: Chloramine-T oxidation method
Q16. Why is specific activity critical in receptor-binding radioligand studies?
- Higher specific activity reduces chemical purity
- It determines the proportion of ligand molecules that carry radioactivity, affecting binding sensitivity and receptor occupancy
- Specific activity controls half-life of the isotope
- Specific activity dictates the color of the radiolabel
Correct Answer: It determines the proportion of ligand molecules that carry radioactivity, affecting binding sensitivity and receptor occupancy
Q17. Which of the following isotopes emits a significant 364 keV gamma photon and is used in therapy and imaging?
- 99mTc
- 131I
- 18F
- 14C
Correct Answer: 131I
Q18. Which experimental design is commonly used to follow temporal changes in metabolism using radiolabeled substrates?
- Continuous infusion without sampling
- Pulse-chase experiment with radiolabeled substrate
- Single time-point mass spectrometry only
- Static immunostaining
Correct Answer: Pulse-chase experiment with radiolabeled substrate
Q19. Which formula gives the effective (biological + physical) half-life of a radionuclide in tissue?
- Te = Tp + Tb
- Te = (Tp * Tb) / (Tp + Tb)
- Te = Tp / Tb
- Te = Tp – Tb
Correct Answer: Te = (Tp * Tb) / (Tp + Tb)
Q20. Which radiation safety principle states that radiation intensity decreases as the square of the distance from the source increases?
- Time principle
- Shielding principle
- Inverse square law
- Contamination control
Correct Answer: Inverse square law
Q21. On discovery of a liquid radiotracer spill in the lab, the immediate appropriate action is to:
- Continue the experiment to prevent data loss
- Evacuate area if required and notify the radiation safety officer immediately
- Wipe with bare hands to remove spill quickly
- Incinerate the contaminated materials
Correct Answer: Evacuate area if required and notify the radiation safety officer immediately
Q22. Which radioisotope is commonly used to label the gamma phosphate of ATP for kinase assays?
- 14C
- 32P
- 99mTc
- 35S
Correct Answer: 32P
Q23. Which factor most strongly affects spatial resolution in autoradiography?
- Color of the emulsion
- Energy of emitted particles and resultant particle range in the medium
- Manufacturer of isotope
- Total activity only
Correct Answer: Energy of emitted particles and resultant particle range in the medium
Q24. Which radioisotope is preferred for diagnostic thyroid imaging due to favorable imaging properties and lower radiation dose compared with 131I?
- 18F
- 123I
- 32P
- 14C
Correct Answer: 123I
Q25. Which laboratory technique is routinely used to assess radiochemical purity of a labelled compound?
- Thin-layer chromatography (TLC)
- pH meter reading
- UV-Vis transmittance only
- Gram staining
Correct Answer: Thin-layer chromatography (TLC)
Q26. Specific activity is commonly expressed in which unit for radiolabelled compounds?
- Moles per liter (mol/L)
- Ci/mmol or MBq/μmol
- g/mL
- Counts per second
Correct Answer: Ci/mmol or MBq/μmol
Q27. For high-resolution autoradiography of oligonucleotide probes, which isotope provides better spatial resolution due to low beta energy?
- 32P
- 35S
- 99mTc
- 131I
Correct Answer: 35S
Q28. Which positron-emitting isotope has an approximate half-life of 20 minutes and is used for rapid synthesis of labeled biomolecules?
- 18F
- 11C
- 14C
- 99mTc
Correct Answer: 11C
Q29. What does radiochemical yield describe in a radiolabeling reaction?
- Total mass of labeled product isolated ignoring radioactivity
- Fraction of starting radioactivity incorporated into the desired radiolabeled product
- Specific activity of the final product only
- Time taken to complete labeling
Correct Answer: Fraction of starting radioactivity incorporated into the desired radiolabeled product
Q30. In the context of radiometal complexes, which process describes loss of label due to transfer of the metal ion to competing ligands in biological media?
- Isotope exchange
- Transchelation
- Radiolysis
- Photobleaching
Correct Answer: Transchelation
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