Rationales A: Where the Wrong Answer Comes From

Item 1 — Answer: 2 Why 2 is right: the potassium is critical and climbing 1.3 mEq/L in a day. The ECG is the assessment that tells you whether the myocardium has responded yet, and it is fast. Why 1 is wrong: correct eventually, wrong first. You notify with an assessment in hand. Why 3 is wrong: useful chart work, but it changes nothing in the next two minutes. Why 4 is wrong: a repeat draw delays care for a value the patient's rhythm can confirm now. The rule: assess, then notify.

Item 2 — Answer: 1, 2, 4, 6 Why 1, 2, 4, 6 are right: calcium 7.2 mg/dL with a normal albumin is true hypocalcemia, and low calcium irritates: perioral and fingertip tingling, a positive Trousseau sign, hyperreflexia, and carpopedal spasm. Why 3 is wrong: constipation and bone pain belong to hypercalcemia. Why 5 is wrong: lethargy and weakness are the sedating picture of a high calcium. The rule: low irritates, high sedates.

Item 3 — Answer: 1 Why 1 is right: an ANC of 320/mm³ is severe neutropenia, and the ANC decides precautions. Why 2 is wrong: the white cell count is the trap. The neutrophil fraction, not the total, carries the defense. Why 3 is wrong: airborne precautions protect others from the client; this client needs protecting from the environment. Why 4 is wrong: bed rest is not a precaution, and once-daily temperature misses the only reliable early sign of infection in a neutropenic client. The rule: the ANC, not the WBC.

Item 4 — Answer: c, e, a, d, b Why c is first: a result on the wrong client is not a result. Why e is second: absent reflexes and a falling respiratory rate change what you say on the call. Why a is third: the provider hears an assessed patient, not a number. Why d is fourth: the read-back is documented with the time, which is the safety step, not a courtesy. Why b is last: the recheck is the evaluation, and it is scheduled, not immediate. The rule: verify, assess, notify, read back, recheck.

Item 5 — Answers: potassium 3.1 expected; bicarbonate 18 expected; sodium 119 critical; calcium 12.8 unexpected Potassium 3.1: stool loss lowers potassium, so it is expected and not critical, which begins below 2.5 mEq/L. Bicarbonate 18: stool carries bicarbonate out, so a metabolic acidosis is expected here. Sodium 119: below 120 mEq/L is critical in any context. Calcium 12.8: high, but diarrhea does not raise calcium, so it is unexpected and needs explaining. It is not critical, which begins above 13.0 mg/dL. The rule: expected means the mechanism explains it. Critical means the threshold is crossed. They are different questions.

Item 6 — Answer: 2 Why 2 is right: potassium fell 1.1 mEq/L to 2.9 mEq/L, below range, with the rhythm consequence. Why 1 is wrong: sodium 141 mEq/L is inside its range and moved 2 mEq/L. Why 3 is wrong: magnesium 1.4 mEq/L is still inside 1.3 to 2.1 mEq/L. Worth watching alongside the potassium, not acted on first. Why 4 is wrong: creatinine 1.2 mg/dL is at the top of its range, and the rising BUN with it is a volume signal, not the first harm. The rule: biggest move plus fastest harm, not biggest number.

Item 7 — Answer: 2 Why 2 is right: the digoxin level is above 2.0 ng/mL, the creatinine says the drug is not clearing, and the potassium of 3.2 mEq/L makes the heart more sensitive to it. Hold, assess the apical pulse, report all three together. Why 1 is wrong: giving a dose on top of a level that is already high is the harm. Why 3 is wrong: correct instinct on potassium, wrong tempo, and it still gives the dose. Why 4 is wrong: holding is right and the timing is nonsense. You cannot repeat a trough after giving the drug. The rule: read the level against the potassium and the kidney.

Item 8 — Answer: 1, 4 Why 1 is right: potassium 2.9 mEq/L is below range and falling. Why 4 is right: BUN 26 mg/dL is above range, up 10 in a day, and the ratio near 22:1 says volume. Why 2 is wrong: sodium 141 mEq/L is normal. Why 3 is wrong: magnesium 1.4 mEq/L is inside its range. Why 5 is wrong: chloride 99 mEq/L is inside its range. Why 6 is wrong: creatinine 1.2 mg/dL is the top of its range, reported with the BUN rather than on its own. The rule: report values that are out of range or moving fast, not values that merely moved.

The three trap patterns

• Right value, wrong action. You named the abnormality and then chose the second-best move. Item 1 and item 7.
• Correct action, wrong order. Every option is something you will do. Only one is first. Item 4.
• A plausible number just outside the tested range. The distractor sits near a threshold you half-remember. Item 6 and item 8.

Page 10 exhibit key

• 0800: pH 7.27 low, HCO₃⁻ 17 low, PaCO₂ 38 normal. Uncompensated metabolic acidosis.
• 1200: pH 7.32 low, HCO₃⁻ 15 low, PaCO₂ 30 low. The lungs are working. Partly compensated metabolic acidosis.
• 1600: pH 7.25 low, HCO₃⁻ 11 low, PaCO₂ 26 low. Still partly compensated, and worse than at 1200 despite harder breathing.
• Anion gap at 1600: 138 − (100 + 11) = 27 mEq/L, against a normal of 8 to 12 mEq/L.
• What the gap changes: a wide gap says unmeasured acid is being produced faster than the lungs can offset it, so the target is the acid production, and the value you follow to prove the treatment is working is the closing gap, not the pH alone.

Margin notethe wrong answers are the syllabus.

→ BOOK 07 · P12 for the items. → BOOK 07 · P10 for the exhibit.

Takeawaymost missed laboratory items are lost on the action, not on the number.

Reference ranges vary between institutions and reference laboratories. Follow local protocol.