Red Ranger wrote:
Phlox finds traces of a genetic flaw that led to T'Pol being more susceptible to Trellium-D, but he has never seen this anomaly in the Vulcan medical database. This would fall under doctor-patient privilege.
Armed with this new information, Phlox speculates that T'Pol's part-Romulan heritage is responsible for this susceptibility to trellium-D.
Remember "Impulse"? Regular Vulcans fell victim to 'the D' quickly enough. It turned a whole ship full of them into zombies. So T'Pol wouldn't need any Romulan heritage to be susceptible to it. It's inherent in all Vulcans.