Concept:
A prescription is a legal document written by a registered prescriber.
Step 1: For a prescription to be valid, it must be authenticated.
Step 2: Authentication is done by the prescriber's signature.
Step 3: Without signature, the prescription may not be legally valid.
Step 4: Diagnosis and cost may be useful, but signature is essential.
\[
\boxed{\text{Prescriber's signature}}
\]