Does masturbating after Fajr prayer in Ramadan, due to the emission of a viscous fluid that prevents urination, invalidate the fast and necessitate making up for the day (qada') or offering expiation (kaffarah)?

Deliberate masturbation is among the things that invalidate the fast and necessitates making up the two days during which it occurred. If masturbation is a necessity for medical treatment, there is no sin, provided that doctors are consulted.