Prolonged labor leads to increased physical and emotional stress on the mother, resulting in maternal exhaustion. This fatigue impairs effective uterine contractions, further delaying labor progression and increasing the risk of complications such as infections and fetal distress. Other options like rapid delivery, polyhydramnios, and fetal macrosomia are not direct clinical features but rather potential causes or unrelated conditions. Hence, maternal exhaustion is the key clinical feature indicative of prolonged labor.

Reference: Williams Obstetrics, Cunningham F.G., 25th Edition.