Final answer:
In the prescription provided, the route of administration is missing, which is essential for the complete instruction on how to take the prescribed medication.
Step-by-step explanation:
The part of the order that is missing is the route of administration for the medication prescribed. An order for medication should include the name of the drug, the dosage, the frequency of administration, and how the medication is to be taken or given (route of administration). In the provided prescription - Doxycycline 750mg tabs q8 hours for 7 days - the route of administration is not specified.