Air Carrier Captain reported a fume event during departure taxi described as 'dirty socks'. Flight returned to gate where maintenance and medical assistance was requested.

Narrative

After having flow from ZZZ1 to ZZZ for a quick turn; we kept the APU running and aircraft cool and comfortable without any issues. Upon pushing back; we received a ground stop notice for ZZZ1 and conferred with Dispatch to continue. After starting #1 engine; Ground Control held us on the ramp for approximately 15 minutes. We had the APU bleed on as well as both packs on (standard southeast taxi configuration). No issues at that time. Upon taxi out clearance; a heavy rain started and with just a small application of power to engine #1 to start the taxi we noticed a faint dirty socks smell as we joined the taxiway out. At that same moment Ground Control notified us that the ground stop was lifted and we started the 2nd engine start sequence. During that time the smell got worse and we were notified by the Flight Attendants of the smell. After shutting down the APU there was absolutely no change and the odor continued to get stronger. We tried to isolate the packs individually to no avail. At that time we told Ground we needed to return to the gate and coordinated with Dispatch and Operations. On the taxi back; the Flight Attendants said the smell was super strong in the aft part of the aircraft and that the aft Flight Attendant was feeling nauseous. I immediately shutdown both packs which started an improvement. The aft Flight Attendant requested to have medical attention at the gate and we arranged for it. Ramp was shut down due to lighting; but Company was able to get us in via a marshaling vehicle. Once at the gate; the aft Flight Attendant got medical attention while we unloaded without any passenger issues. Afterwards; all the Flight Attendants sought to be checked out by a medical facility.Purely a mechanical issue that occasionally plagues the airbus. No improper action lead to this issue. Airbus has had these issues for far too long to not have found a permanent fix or redesign. At what point will health concerns outweigh cost?