Differential diagnosis formulation is the systematic process of generating, prioritizing, and refining a list of possible diagnoses based on clinical data. For the nurse practitioner, this skill represents the core of autonomous clinical decision-making, distinguishing advanced practice from task-oriented care. The cognitive process employs dual-process theory: System 1 (intuitive pattern recognition) rapidly generates diagnostic hypotheses based on illness scripts stored in clinical memory, while System 2 (analytical reasoning) systematically evaluates each hypothesis against clinical data using hypothetico-deductive reasoning. Expert clinicians seamlessly integrate both systems — recognizing common patterns quickly while engaging analytical reasoning for atypical or complex presentations. The differential diagnosis process begins with problem representation: distilling the clinical presentation into a concise semantic statement that captures the key features (e.g., 'acute-onset severe headache in a 45-year-old hypertensive female with neck stiffness'). Effective problem representation activates relevant illness scripts and narrows the differential before systematic analysis. Bayesian reasoning underlies differential prioritization: pre-test probability (disease prevalence in the patient's demographic) is modified by the sensitivity and specificity of clinical findings (likelihood ratios) to generate post-test probability. The NP must also apply...