Diagnostic Formulation and Reasoning
From findings to a defensible differential
Formulation stations ask you to synthesise history and examination into a problem list, a ranked differential and a plan. Examiners reward structured reasoning, not a lucky single diagnosis. Start with a one-line summary: age, relevant background, key symptoms and signs, and the central problem.
Concepts to demonstrate
Build a problem representation, then generate a differential using anatomical, physiological or systems-based reasoning. Prioritise by likelihood and by danger, explicitly naming the "cannot miss" diagnoses. Link each diagnosis to supporting and refuting features from the data. Identify the investigations that would discriminate between them, and explain why each test is chosen.
Traps that cost marks
- Anchoring on the first diagnosis and ignoring discordant features.
- Listing conditions without justification or ranking.
- Omitting serious diagnoses because they are uncommon.
- Proposing investigations without stating the question each answers.
- Forgetting to consider drug effects and comorbidity.
Worked rule of thumb
Use a two-axis ranking: probability and harm if missed. Place the most dangerous plausible diagnosis high even when less likely, and state how you will exclude it. For example, a young woman with pleuritic chest pain and a recent long-haul flight: rank pulmonary embolism high on the danger axis despite low absolute probability, justify with risk factors, and propose appropriate imaging and interim management.
Finish by summarising the plan in categories: investigations, immediate treatment, safety-netting, and follow-up. Clear, justified reasoning scores better than a correct diagnosis delivered without explanation.