Which sign is characterized by concavity of the frontal bones and is associated with spina bifida?

• A. Lemon sign
• B. Keyhole sign
• C. Hypotelorism
• D. Proboscis

Answer: (A)

A concave frontal skull contour on a fetal head ultrasound, known as the lemon sign, is classic for open neural tube defects such as spina bifida. This sign appears because open spina bifida, often with associated Chiari II malformation, causes downward displacement and crowding of intracranial structures, which makes the frontal bones appear scalloped and give the skull a lemon-like shape. This marker is most helpful in the midtrimester screening for neural tube defects.

The other signs point to different conditions: the keyhole sign relates to urinary tract obstruction (posterior urethral valves), while hypotelorism and proboscis are facial/anterior brain anomalies seen with holoprosencephaly, not spina bifida.