Stage 3 — Prioritize hypotheses. The client is a 65-year-old male on postoperative day 7 after right total knee arthroplasty whose prophylactic enoxaparin stopped 2 days ago. He has sudden sharp right-sided chest pain that worsens with inspiration and shortness of breath. BP is 92/58 in the right arm and 94/60 in the left, HR 124, RR 28, SpO2 88% on room air, and temperature 36.7°C (98.1°F) with no recent fever or cough. He is diaphoretic and anxious, and the right calf is tender along the deep veins. The 12-lead ECG shows sinus tachycardia without ST-segment elevation or depression, and breath sounds are clear bilaterally. Laboratory and imaging results are pending. Complete the diagram by selecting the condition the client is most likely experiencing, 2 actions the nurse should take to address that condition, and 2 parameters the nurse should monitor to assess the client's progress.

Quick answer
Acute pulmonary embolism. Continue oxygen with continuous cardiopulmonary monitoring; notify the provider and prepare for CT pulmonary angiography. Monitor oxygen saturation, and blood pressure with heart rate.

Why this is correct
Major orthopedic surgery plus stopped prophylaxis creates high VTE risk, and deep-vein calf tenderness suggests a source. Sudden pleuritic pain, hypoxemia with clear lungs, tachycardia, and borderline hypotension fit an embolus blocking pulmonary blood flow. The ECG without ST changes, equal arm pressures, and absent fever and cough lower suspicion for the main competing emergencies.

Easy analogy or mental picture
A clot from the leg travels like a cork into the lung's pipes. Air still moves in and out (clear breath sounds), but blood cannot reach the air, so oxygen falls and the heart races to push past the cork.

Memory hook
"New leg joint + stopped blood thinner + sudden pleuritic dyspnea + clear lungs = think PE."

NCLEX decision rule
When a high-risk postoperative client has sudden hypoxemia with clear lungs, support oxygenation, escalate for definitive imaging, and trend oxygenation and hemodynamics while treatment is prepared.

Why the other choices are wrong
ACS is tempting with chest pain and diaphoresis, but the ECG has no ST changes and the pain is pleuritic, and nitroglycerin could worsen borderline hypotension; serial ST segments track ischemia, not PE. Aortic dissection is ruled against by equal arm pressures, so bilateral pressures do not track this problem. Pneumonia would bring fever, cough, or focal crackles, so cultures and the WBC count are not the priority. A prophylactic enoxaparin dose is not treatment for a suspected PE; therapeutic anticoagulation is prescribed after assessment and bleeding-risk review.