A 6-year-old boy with heart failure weighs 18 kg and is started on spironolactone 1 mg/kg daily in two divided doses. The spironolactone solution is 25 mg/5 mL. How many millilitres are needed for a 28-day supply, to the nearest millilitre?

• A. 101 mL
• B. 90 mL
• C. 120 mL
• D. 75 mL

Answer: (A)

The amount is based on weight to determine the daily amount of drug, then converted to a volume using the solution’s concentration, and finally scaled to 28 days.

First, the daily dose is 1 mg per kg for a child weighing 18 kg: 1 mg/kg × 18 kg = 18 mg per day. The solution is 25 mg in 5 mL, which is 5 mg per mL. So the daily volume needed is 18 mg ÷ 5 mg/mL = 3.6 mL per day (even though it’s given in two divided doses, the total daily volume is what matters for the 28-day supply).

For 28 days, total volume = 3.6 mL/day × 28 days = 100.8 mL, which rounds to 101 mL.

Therefore, about 101 mL are needed for a 28-day supply.