<qa>  What is the randomized, double blind, single-site, 4 period crossover study?This will be a randomized, double blind, single-site, 4 period crossover study in healthy male and female subjects to determine if droxidopa administered as a single therapeutic 600 mg and a single supratherapeutic 2000 mg dose delays cardiac repolarization as determined by the measurement of QTc interval. Study drug droxidopa, placebo, and moxifloxacin will be administered in a double blind, double dummy manner. The central ECG laboratory eResearch Technology, Inc, Philadelphia, Pennsylvania will be blinded to treatment. 