After spending as much as half of her 11 years bedridden with a chronic blood disorder, the last place Liza Rybentseva expected to end up was on stage at the Moscow Conservatory with a world famous soprano. But that's exactly where she is scheduled to be Sept. 5, alongside renowned Spanish diva Montserrat Caballe and her rising-star daughter, Montserrat Marti. She won't be there just to watch and listen. As her tiny, pale hands coax the first mellow tones of Mendelssohn's concerto from her violin, it becomes clear why the concert organizers wanted her. ""I am so happy that I will perform with such a famous singer,"" Liza said weakly as she cradled her violin in the austere, one-room apartment she shares with her parents in northwestern Moscow. Barring another bout with the effects of neutropenia, a rare blood disorder that leaves her vulnerable to frequent bacterial infections, the gala concert is likely to be the high point of her life so far.