Which feature indicates distribution phase in a two-compartment model?

• A. A steep initial decline in the log concentration-time plot (alpha phase)
• B. A linear decline in concentration over time
• C. A constant concentration after dosing
• D. A plateau during the beta elimination phase

Answer: (A)

In a two-compartment model, the distribution phase is the early, rapid movement of drug from the central to the peripheral compartment. This shows up as a steep drop in the log concentration–time plot, known as the alpha phase. The slope is much steeper than the later elimination phase, which reflects slower removal of the drug from the body (beta phase). So the steep initial decline on the semi-log plot is the telltale sign of distribution.

The other options don’t fit this pattern: a linear decline isn’t characteristic of the two-phase distribution and elimination seen on a semi-log plot; a constant concentration after dosing isn’t expected in either phase; and a plateau during elimination would imply no further decrease, which isn’t the case in first-order elimination.