Final answer:
William's absence of nocturnal penile tumescence could be an indicator of erectile dysfunction (ED), a condition that prevents a man from obtaining or maintaining an erection due to various physical or psychological factors.
Step-by-step explanation:
William's lack of penile erections while sleeping could indicate erectile dysfunction (ED), which is a disorder characterized by the regular and repeated inability of a sexually mature male to obtain or maintain an erection. Nocturnal penile tumescence (NPT) is the normal occurrence of erections during REM sleep and its absence can be a sign of erectile dysfunction. It is important to differentiate this from priapism, a condition of prolonged erection, and from impotence, which is often used synonymously with ED but can also refer to other reproductive issues. ED can be associated with several factors such as diabetes, vascular disease, psychiatric disorders, prostate disorders, drug use, or low testosterone levels, all of which can lead to problems with the normal flow of blood or nervous control of penile engorgement.