Final answer:
A medical assistant should document a patient's vital signs under the 'objective data' section of medical records, as this section is reserved for measurable and observable data, in contrast to subjective patient history or complaints.
Step-by-step explanation:
Vital signs are critical, objective indicators of a patient's physiological state, and are routinely documented in a patient's medical records. These signs include body temperature, heart rate, breathing rate, and blood pressure. They are not part of the patient's subjective experience, like the chief complaint, nor are they part of the patient's personal medical history or allergy record.
Therefore, a medical assistant would document a patient's vital signs under objective data, which is the section that holds measurable data observed by the clinician. This section is distinct from patient history, allergy record, and chief complaint because it focuses specifically on quantifiable data, rather than patient-provided information or subjective symptoms.