**Core Concept**
The "Lemon sign" and "Banana sign" are ultrasound markers used in antenatal diagnosis to predict the presence of spina bifida in the fetus. These signs are observed due to the abnormal development of the fetal skull, which affects the shape of the head.

**Why the Correct Answer is Right**
The "Lemon sign" is characterized by a flattened posterior fossa, giving the fetal head a lemon-like appearance. This is due to the herniation of the cerebellum and the medulla oblongata into the foramen magnum, resulting from the failure of the cranial vault to close properly. The "Banana sign" is observed when the cerebellum herniates through the foramen magnum, causing the normal banana-shaped cerebellum to become curved and deformed. This is often associated with the presence of a Chiari II malformation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the antenatal ultrasound markers mentioned in the question.
**Option B:** This option is incorrect as the "Lemon sign" and "Banana sign" are not associated with other conditions such as anencephaly or encephalocele.
**Option C:** This option is not correct as the "Lemon sign" and "Banana sign" are not indicative of a normal fetal head shape.

**Clinical Pearl / High-Yield Fact**
The "Lemon sign" and "Banana sign" are often used in combination with other ultrasound markers, such as the presence of a nuchal fold thickness greater than 6 mm, to increase the accuracy of spina bifida diagnosis.

**Correct Answer: C. Spina bifida.**