Final answer:
In a medical office, the highest priority mail is patient-related, as it contains important information for patient care that must be processed quickly.
Step-by-step explanation:
When a medical assistant receives postal mail delivery at the office and sorts it, the type of mail that receives top priority is D) Patient-related mail. This type of mail often contains sensitive and important information that could affect patient care and must be processed as quickly as possible to ensure that the medical staff can address patient needs promptly. Examples of patient-related mail include lab results, referral letters, and communication from specialists or insurance companies concerning patient care and treatment plans.