Clinicians working within primary care services tend to do so in isolation and without direct access to investigative resources. This places emphasis on the clinician’s abilities to perform a focused clinical examination and employ sound clinical decision-making processes to reach a definitive diagnosis. This article looks at the case history of a patient presenting to a GP out-of-hours service with an acute, undifferentiated illness, and explores the decisionmaking processes used by the author, a trainee advanced nurse practitioner at the time, to support a working diagnosis and justify a clinically suitable management plan.