For a drug with a large Vd, the distribution process can be rate-limiting, leading to a prolonged distribution phase.

• A. The distribution phase is rapid and not rate-limiting.
• B. The distribution phase is rapid and rate-limiting.
• C. Distribution is the rate-limiting step; the initial decline reflects tissue distribution.
• D. Elimination is always rate-limiting.

Answer: (C)

When a drug has a large volume of distribution, most of the drug is stored in tissues rather than in plasma, so moving between plasma and tissues becomes the slow, controlling step. In a two-compartment view, the early drop in plasma concentration after administration is dominated by distribution into tissues rather than by elimination. If this distribution process is slow, it becomes the rate-limiting step that shapes the overall pharmacokinetic profile, producing a prolonged distribution phase. Once distribution reaches equilibrium, what remains in plasma declines mainly due to elimination. Thus the correct idea is that distribution is the rate-limiting step, and the initial decline in plasma reflects tissue distribution.