Leptospirosis is difficult to diagnose because of protean nonspecific clinical manifestations and the lack of rapid, actionable laboratory testing. A previously healthy woman who presented with nausea is described in the present study. She had modestly elevated liver function tests, an unusual pretibial rash, and no fever. She had recently swum in a waterfall in Costa Rica. After the clinicians proposed a leptospirosis diagnosis to the patient, an artificial intelligence program was selected for use by the clinicians to enhance confidence in proposing a rare and unknown diagnosis. The program independently produced a differential diagnosis that led with leptospirosis. Presumptive doxycycline administration led to symptom resolution. Blood (but not urine) Polymerase chain reaction (PCR) test for the LipL32 gene and a novel antibody detection test confirmed leptospirosis. This case represents an unusual clinical presentation of leptospirosis, including the absence of fever, the presence of a pretibial rash, and isolated, mildly elevated bilirubin and transaminases as sole laboratory abnormalities, and highlights difficulties associated with making a rapid, actionable diagnosis.