Advanced diagnostic reasoning by the NP integrates pattern recognition (System 1 thinking — fast, intuitive, experience-based) with analytical reasoning (System 2 thinking — slow, deliberate, hypothesis-driven). The hypothetico-deductive method is the foundational clinical reasoning model: generate initial hypotheses from the chief complaint and early cues → gather targeted data through focused history and examination to test hypotheses → revise hypothesis list based on findings → select the most probable diagnosis. Bayesian reasoning adjusts the probability of each diagnosis based on sequential findings: pre-test probability (prevalence + clinical presentation) × likelihood ratio (sensitivity/1-specificity for positive test) = post-test probability. Key cognitive biases that impair diagnostic reasoning: anchoring (fixating on an initial diagnosis despite contradicting evidence), premature closure (accepting a diagnosis before adequate verification), availability bias (overweighting diagnoses that come easily to mind — recent case or dramatic presentation), confirmation bias (seeking data that confirms while ignoring data that refutes the working diagnosis), and diagnosis momentum (accepting a prior clinician's diagnosis without independent verification). The illness script is the expert clinician's mental model containing pathophysiology, epidemiology, time course, clinical features, and enabling...