The Medical Supplies Mass Effect 3 Citadel mission can be completed after Kallini: Ardat-Yakshi Monastery side-mission.
To start the quest, talk to your doctor on Normandy and she will tell you that she ran out of medical supplies.
When to start
After Priority: The Citadel 2
Enhanced medi-gel – If the doctor is Dr. Chakwas
Increased medi-gel capacity – It the doctor is Dr. Michel
After you start the quest, go to the Docks: Holding Area and find Tactus.
Convince him to close a deal with you, and then report to your doctor.
NOTE: To complete this quest you need a specific amount of Paragon/Renegade Points.