A 4-year-old girl with tonsillitis is prescribed phenoxymethylpenicillin 125 mg QID for 10 days. If the suspension is 250 mg/5 mL, what is the total quantity in millilitres needed for the entire course?

A 4-year-old girl with tonsillitis is prescribed phenoxymethylpenicillin 125 mg QID for 10 days. If the suspension is 250 mg/5 mL, what is the total quantity in millilitres needed for the entire course?

• A. 100 mL
• B. 50 mL
• C. 75 mL
• D. 125 mL

Answer: (A)

The key idea is converting the prescribed mg dose into a suspensions volume using its concentration, then scaling by dosing frequency and treatment duration. The suspension has 250 mg in 5 mL, which is 50 mg per mL. Each dose is 125 mg, so the volume per dose is 125 mg ÷ 50 mg/mL = 2.5 mL. With four doses per day, the daily volume is 2.5 mL × 4 = 10 mL. Over 10 days, total volume = 10 mL/day × 10 days = 100 mL. Therefore, the total quantity needed is 100 mL.

The key idea is converting the prescribed mg dose into a suspensions volume using its concentration, then scaling by dosing frequency and treatment duration. The suspension has 250 mg in 5 mL, which is 50 mg per mL. Each dose is 125 mg, so the volume per dose is 125 mg ÷ 50 mg/mL = 2.5 mL. With four doses per day, the daily volume is 2.5 mL × 4 = 10 mL. Over 10 days, total volume = 10 mL/day × 10 days = 100 mL. Therefore, the total quantity needed is 100 mL.