Two patients undergo audiometric testing. Patient A, a 28-year-old with sudden sensorineural hearing loss and a positive Romberg test, shows a bone conduction threshold of 45 dB at 2 kHz and air conduction of 50 dB at the same frequency. Patient B, a 56-year-old with progressive bilateral hearing loss, shows bone conduction of 35 dB and air conduction of 55 dB at 2 kHz. Which finding best distinguishes conductive hearing loss from sensorineural hearing loss in these audiograms?