What is the primary use of Sildenafil citrate (Viagra)?

Sildenafil citrate, commonly known by its brand name Viagra, is primarily used for the treatment of erectile dysfunction. This medication works by inhibiting phosphodiesterase type 5 (PDE5), which leads to an increase in blood flow to the penis when sexually stimulated. This effect allows for a sufficient erection in individuals who may have difficulty achieving or maintaining it.

Among the options provided, the most accurate primary application aligns best with option B, relating to treating conditions affecting sexual function rather than the other choices listed. Chronic pain relief, cardiac arrhythmias, and sleep apnea are medical conditions that do not benefit from sildenafil’s mechanism of action. While altitude sickness, cardiac arrhythmias, and sleep apnea are important medical concerns, sildenafil's established use and primary indication do not encompass those conditions, reinforcing its primary role in addressing erectile dysfunction.