You are given MICs for two antibiotics: Drug A inhibits step 1 of a biosynthetic pathway (MIC = 8 µg/mL alone) and Drug B inhibits step 2 (MIC = 4 µg/mL alone). In combination, inhibitory concentrations are 2 µg/mL for A and 1 µg/mL for B. Calculate the fractional inhibitory concentration (FIC) index and interpret whether the interaction is synergistic, additive/indifferent, or antagonistic.

• A. FIC index = 1.00, indicating indifference because the combined concentrations are equivalent to the sum of single-agent effects.
• B. FIC index = 2.00, indicating antagonism because each drug requires twice its single-agent MIC in combination.
• C. FIC index = 0.50, indicating synergism (combination allows pathway shutdown at lower doses).
• D. FIC index = 0.25, indicating extreme synergy such that doses can be reduced by a factor of four across the board without loss of activity.

Answer: (C)