Final answer:
FUO, which stands for Fever of Unknown Origin, is the abbreviation used when the cause of a patient's symptoms, notably fever, is undetermined after thorough investigation.
Step-by-step explanation:
The abbreviation used to document cases in which the cause of a patient's symptoms are unknown is b) FUO, which stands for Fever of Unknown Origin. This term is utilized when a physician is unable to determine the cause of a patient's fever after extensive evaluation. It differs from a diagnosis of a UTI, which is a Urinary Tract Infection, where the cause of the symptoms is typically known and often bacterial in origin.