Does yellow discharge around the anus after defecation nullify ablution (wudu') and invalidate prayer (salat), and is it necessary to change underwear despite no trace of the discharge on it?

What exits the anus by way of discharge invalidates ablution and necessitates Istinja' (washing the private parts) due to its impurity, unless it is continuous, in which case it is considered سلس ( سلس البول, سلس البراز: incontinence). Changing clothes is not obligatory unless these discharges have soiled them, in which case they must be changed or washed, because one of the conditions for the validity of prayer is purification from impurity on the garment, body, and place.