A 14-year-old patient weighs 40 kg and is to receive methotrexate at 50 mg/m^2. What is the dose (to the nearest 0.5 mg)?

• A. 52.5 mg
• B. 72.5 mg
• C. 62.5 mg
• D. 42.5 mg

Answer: (C)

Dosing by body surface area means you multiply the dose per square meter (mg/m^2) by the patient’s BSA in square meters. For a 40 kg, typical adolescent patient, the body surface area is about 1.25 m^2. Using Mosteller-style estimation, a height around 140 cm gives BSA ≈ sqrt(140 × 40 / 3600) ≈ sqrt(1.56) ≈ 1.25 m^2. So the total dose = 50 mg/m^2 × 1.25 m^2 = 62.5 mg, which is already at the nearest 0.5 mg. That matches the chosen option.

If you tried different BSA values (for example around 1.05 m^2 or 1.45 m^2), you’d get 52.5 mg or 72.5 mg respectively, which aren’t typical estimates for a 40 kg, adolescent patient.