Take a recent case. A 28-year-old nurse presents at 3am with sudden left-sided pleuritic chest pain and breathlessness. She started the combined oral contraceptive pill six weeks ago. Her heart rate is 112, respiratory rate 24, oxygen saturation 93 percent on air, and blood pressure 118 over 72. Her chest is clear. What is the most likely diagnosis, and what is the single best initial investigation?

The answer is pulmonary embolism. Pleuritic pain, tachycardia, hypoxia, and recent oral contraceptive use give a Wells score of at least 4.5, and CTPA is the gold-standard investigation. A D-dimer could be used when the Wells score is 4 or below, but is less useful at this level of probability.

Each case comes with teaching points that stay with you longer than the answer. The combined oral contraceptive pill is a significant risk factor for venous thromboembolism, so always ask about hormonal contraception. A clear chest with hypoxia should raise suspicion for PE.

A Wells score of 5 or more is high probability, so go straight to CTPA and skip the D-dimer. And tachycardia, not the classic S1Q3T3, is the most common ECG finding in PE. Small, memorable lessons like these are what build clinical reasoning over time.