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MCAT · Question of the Day

MCAT Question of the Day

A fresh daily question to build accuracy, reinforce recall, and turn practice into a steady habit.
Monday, September 21, 2026

A PET center compares two positron-emitting tracers. Tracer X has λ=6.30×10−3 min−1\lambda=6.30\times10^{-3}\ \text{min}^{-1}; Tracer Y has λ=1.05×10−2 min−1\lambda=1.05\times10^{-2}\ \text{min}^{-1}. Both decay by β+\beta^+ emission and are injected at the same initial activity. Based on the decay model, what outcome is most likely?

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Question of the Day

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A PET center compares two positron-emitting tracers. Tracer X has λ=6.30×10−3 min−1\lambda=6.30\times10^{-3}\ \text{min}^{-1}; Tracer Y has λ=1.05×10−2 min−1\lambda=1.05\times10^{-2}\ \text{min}^{-1}. Both decay by β+\beta^+ emission and are injected at the same initial activity. Based on the decay model, what outcome is most likely?

  1. Tracer Y will have a shorter half-life and its activity will drop faster than Tracer X (correct answer)
  2. Tracer Y will have a longer half-life and its activity will drop slower than Tracer X
  3. Both tracers will have identical half-lives because they decay by the same mode
  4. Tracer X will drop faster because a smaller decay constant implies faster decay

Explanation: This question tests understanding of nuclear decay and radioactivity (4E). Radioactive decay involves the transformation of an unstable nucleus into a more stable one, often emitting particles or radiation. Here, two tracers with different decay constants undergo β⁺ decay. Choice A is correct because Tracer Y has a larger λ, implying a shorter half-life and faster decay. Choice B is incorrect as it reverses the relationship between λ and half-life. When assessing decay scenarios, ensure the decay type aligns with given isotopic characteristics and half-life data. Cross-check decay products and process assumptions.