Final answer:
The bulbourethral gland is the accessory gland responsible for cleansing the urethra before ejaculation, neutralizing acidic residue and providing lubrication.
Step-by-step explanation:
The accessory gland responsible for cleansing the urethra prior to ejaculation is the bulbourethral gland, also known as Cowper's gland. This gland produces a secretion that neutralizes any acidic residue left over from urine, thereby cleansing the urethra and preparing it for the passage of semen. In addition to its cleansing action, the fluid from the bulbourethral gland also provides lubrication for the urethra.