Which nerve is responsible for sympathetic innervation of the bladder?

The hypogastric nerve is responsible for sympathetic innervation of the bladder. This nerve is part of the autonomic nervous system and specifically carries sympathetic fibers that help regulate the bladder's function. When stimulated, the hypogastric nerve induces bladder relaxation and increases the capacity of the bladder by inhibiting detrusor muscle contractions. This mechanism plays a crucial role in the storage phase of bladder function, allowing the bladder to fill up without involuntary emptying.

In contrast, the pelvic nerve primarily carries parasympathetic fibers, which facilitate bladder contraction during urination. The phrenic nerve is primarily involved in diaphragm control, and the vagus nerve has diverse functions but does not innervate the bladder sympathetically. Therefore, the hypogastric nerve is the sole nerve that mediates sympathetic responses regarding bladder control, making it the correct choice.