Which instrument is specifically used for examining the bladder?

The cystoscope is the instrument specifically designed for examining the bladder. This specialized endoscope allows for direct visualization of the bladder's interior, facilitating the diagnosis and treatment of various urinary tract conditions. The procedure typically involves the insertion of the cystoscope through the urethra, enabling healthcare providers to inspect the lining of the bladder, take biopsies, and perform minor surgical procedures if necessary.

The other instruments listed serve different purposes. A filiform catheter is used primarily for navigating and gaining access within the urethra, while metal sounds are instruments used for sounding and dilating the urethra but do not provide visual examination capabilities. Hydrocele refers to a fluid-filled sac around the testicle, commonly unrelated to bladder examination. The cystoscope is thus essential in urology for its ability to provide clear, visual access to the bladder, making it the correct choice for this question.