What term denotes the time required for an organism to eliminate half of an administered dose of a substance through normal processes?

• A. Biologic half-life
• B. Attenuation
• C. Detective Quantum Efficiency
• D. Spatial resolution

Answer: (A)

The concept being tested is biological half-life—the time it takes for the body to eliminate half of a drug or substance through normal metabolic and excretory processes. In pharmacokinetics, half-life describes how quickly elimination occurs. After one half-life, about 50% remains; after two, about 25%; after three, about 12.5%, and so on. The actual duration depends on how quickly the body clears the substance (clearance) and how extensively the substance distributes in body tissues (volume of distribution). A useful relation is t1/2 ≈ 0.693 × Vd / CL, which helps connect the concept to underlying physiology: greater distribution or slower clearance lengthens the half-life, meaning the substance stays in the body longer.

Attenuation, Detective Quantum Efficiency, and Spatial resolution are terms from imaging physics that describe signal loss in tissues, detector efficiency, and the ability to distinguish fine detail in an image, respectively. They do not describe the body's time-based elimination of a substance.