A dose is 25 mg; stock solution is 100 mg/5 mL. How many mL should be administered?

• A. 0.5 mL
• B. 2 mL
• C. 1.25 mL
• D. 5 mL

Answer: (C)

This question hinges on converting a concentration to a per-milliliter value and then using that to find the volume needed to deliver the desired dose. The stock solution provides 100 mg in 5 mL, which is 20 mg per 1 mL. To deliver 25 mg, you divide the dose by the concentration: 25 mg ÷ 20 mg/mL = 1.25 mL. You can also set up a proportion: 25 mg × (5 mL/100 mg) = 1.25 mL. So the amount to administer is 1.25 mL. Quick checks: 0.5 mL would give 10 mg, 2 mL would give 40 mg, and 5 mL would give 100 mg, none of which match the target dose.