Rationales A — the setup, the answer, and the near-miss
Six paired rows, each correct cancellation chain on the left with cancelled labels struck and the surviving unit boxed, the flawed version on the right with broken arcs that stop short of meeting, and a narrow center column naming the character of each error.
Item 1 — morphine sulfate 8 mg, supply 10 mg/mL
8 mg × (1 mL / 10 mg) = 8 ÷ 10 = 0.8 mL Wrong answer 1.25 mL: the supply ratio was inverted, giving 10 ÷ 8. The tell is an answer above one milliliter for a dose below the supply strength. Wrong answer 8 mL: the supply was ignored and the milligram figure was copied into a milliliter box.
Item 2 — cephalexin 375 mg, supply 250 mg per 5 mL
375 mg × (5 mL / 250 mg) = 1875 ÷ 250 = 7.5 mL Wrong answer 1.5 mL: 375 was divided by 250 and the work stopped before multiplying by the 5 mL quantity. Drop the quantity and there are no milliliters to cancel into. Wrong answer 18.75 mL: 1875 was divided by 100 instead of 250, a misread strength.
Item 3 — 1500 mL over 12 hours by pump
1500 mL ÷ 12 hr = 125 mL/hr Wrong answer 12.5 mL/hr: a one-place decimal slip. At that rate the bag would take five days to empty. Reasonableness catches this instantly. Wrong answer 750 mL/hr: the volume was divided by 2 rather than by 12, a keypad error. No peripheral maintenance infusion runs at 750 mL/hr, so reasonableness rejects it on sight.
Item 4 — 100 mL over 30 minutes, 20 gtt/mL
(100 mL × 20 gtt/mL) ÷ 30 min = 2000 ÷ 30 = 66.67 = 67 gtt/min Wrong answer 33 gtt/min: divided by 60 minutes out of habit when the stem said 30. Read the time in the stem, not the time in your memory. Wrong answer 66 gtt/min: 66.67 was truncated rather than rounded. Drops round to the nearest whole drop.
Item 5 — cefazolin 25 mg/kg, child 44 lb, supply 250 mg/mL
44 lb × (1 kg / 2.2 lb) = 20 kg. 20 kg × 25 mg/kg = 500 mg. 500 mg × (1 mL / 250 mg) = 2 mL Wrong answer 4.4 mL: 44 was treated as kilograms, giving 1100 mg. That is page 9's trap appearing inside a live item. Wrong answer 9.7 mL: the weight was converted in the wrong direction. 44 × 2.2 = 96.8 kg, then 2420 mg, then 9.68 mL. Kilograms are the larger unit, so the number had to shrink.
Item 6 — insulin 6 units/hr, bag 100 units in 100 mL
100 units ÷ 100 mL = 1 unit/mL. 6 units/hr × (1 mL / 1 unit) = 6 mL/hr Wrong answer 60 mL/hr: the concentration was misplaced by a factor of ten. On a high-alert infusion this is precisely the difference the independent double check exists to catch. Wrong answer 0.6 mL/hr: the same factor of ten in the other direction, from reading the bag as 1000 units in 100 mL, giving 10 units/mL and 6 ÷ 10.
Rounding note: item 4 rounds up from 66.67. Item 1 stays at one decimal place. Round once, at the end, to the unit the device can actually deliver.
Margin noteevery wrong answer here is somebody's real shift. → BOOK 06 · P9
Takeawaywrong answers cluster at inversions, at factors of ten, and at the pound, and each cluster has a different fix.