The fractional rate of exit from tissue, denoted kT, is defined as:

• A. rate of exit divided by amount in tissue
• B. amount in tissue divided by rate of exit
• C. rate of entry into tissue divided by amount in tissue
• D. time constant of distribution

Answer: (A)

The fractional rate of exit from tissue describes how quickly drug leaves tissue relative to how much is present there. In a first-order exit process, the amount in tissue A_T changes as dA_T/dt = -k_T A_T. The rate of exit is the positive quantity -dA_T/dt, which equals k_T A_T. Solving for the fractional rate constant gives k_T = (rate of exit) / (amount in tissue). This yields units of 1/time and captures the idea that the exit speed scales with how much drug is in the tissue. The alternative ideas mix up what is being related or refer to a different concept (entry into tissue or a general distribution time constant), so they don’t define the fractional rate of exit.