LINK....FREE REGISTRATION REQUIREDIf the fourth case of the normally extremely rare infection is confirmed, it would make it more difficult for the drug to return to the market, doctors and analysts said. The new case might indicate the drug is more dangerous than previously thought because it appears the fourth patient might have taken the drug for a substantially shorter time than the other three.

Mr. McGlynn of Elan said the patient with the new possible case was not someone who had participated in clinical trials of the drug, as the other three confirmed P.M.L. patients had done. That would suggest the person would have received only a few monthly doses at most, since the drug was not generally available before November.

"If this someone who got three or four doses that would be a major concern," said Dr. Patricia K. Coyle, a professor and the acting chairwoman of neurology at SUNY Stony Brook.

The first two patients had taken Tysabri, in combination with Avonex, another multiple sclerosis drug from Biogen for more than two years. The third patient, who had Crohn's disease, had received 8 doses of Tysabri over 18 months and had not taken Avonex