Which statement best describes unbound drug exposure when fu rises?

• A. The unbound exposure decreases
• B. The unbound exposure remains unchanged
• C. The unbound exposure increases
• D. The unbound exposure becomes unpredictable

Answer: (C)

When the fraction unbound rises, more of the drug in plasma is present as the free, active form. The free concentration in plasma, C_free, equals fu times the total plasma concentration (C_total). If C_total stays the same, increasing fu directly increases C_free, so unbound exposure rises. The other options would imply less, unchanged, or unpredictable free exposure, which doesn’t align with the direct relationship between fu and the amount of unbound drug available.