An experienced nurse who has just done a cervical exam on your patient calls you to Labor and Delivery. She reports that she believes the vertex is not presenting. You perform a vaginal examination and confirm the presenting part is not the vertex. In considering malpresentations, which of the following portions of the fetus that is foremost, or presenting, in the birth canal is not compatible with a vaginal delivery? a. Compound
b. Shoulder
c. Brow
d. Face mentum anterior

The correct answer is - B. shoulder

Explanation:

The position of a fetus is facing towards the back of the mother in normal condition with the face and body angled to one side and the neck flexed, and the standard presentation is headfirst.

An abnormal position is facing forward instead of rearward, and abnormal presentations include four parts that are face, brow, breech, and shoulder.

In this particular case, all three presentations are compatible with vaginal delivery in the birth canal except the shoulder as it is not compatible according to the malpresentation described in the question.

Thus, the correct answer is - B. shoulder