You observe in vitro that adding Drug Q to Drug R reduces the MIC of Drug R 8-fold. To determine if the synergy is due to increased uptake of R caused by Q disrupting the membrane, which experimental follow-up would best test this hypothesis?

• A. Measure only the serum protein binding of Drug Q in animal plasma and assume that any change in binding correlates directly with bacterial uptake of Drug R, which would confirm uptake-mediated synergy.
• B. Perform a disk diffusion test with each drug separately and in combination on a lawn of fungi to evaluate cross-kingdom activity; increased zone sizes in fungi prove the drugs disrupt bacterial membranes and increase uptake.
• C. Measure intracellular concentration of Drug R with and without pre-treatment with Drug Q, and perform time-kill assays; an increase in intracellular R with enhanced killing supports uptake-mediated synergy.
• D. Skip laboratory assays and immediately change clinical therapy based on the MIC reduction alone, because a drop in MIC always indicates increased uptake and therefore no further testing is required.

Answer: (C)