A 25-year-old women whose antenatal period was uncomplicated is in labour. She has a single foetus in cephalic presentation. The head is not engaged. The foetal heart rate is 150 beats per minute. The cervical dilatation is 5 cm, the membranes are absent and the pelvis is adequate. It is decided to perform a caesarean section immediately.Which one of the following findings is the most likely cause for this decision?