**Core Concept**
The "banana and lemon sign" refers to a characteristic sonographic appearance of the cerebellum and posterior fossa in fetal ultrasound, indicative of spina bifida. This sign is named for the resemblance of the cerebellum to a banana and the cisterna magna to a lemon.

**Why the Correct Answer is Right**
The banana sign is observed due to the abnormal location of the cerebellum, which is displaced downward and laterally, resulting in a banana-like appearance. This displacement occurs because of the Chiari II malformation associated with spina bifida, where the cerebellum herniates into the foramen magnum. The lemon sign is seen in the anterior curvature of the frontal bones due to increased intracranial pressure and the resultant molding of the skull bones.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not describe a characteristic sonographic appearance of the cerebellum or posterior fossa in fetal ultrasound.
**Option B:** This option does not accurately describe the banana and lemon sign, which are specific features of spina bifida.
**Option C:** This option does not describe a relevant sonographic appearance in the context of fetal ultrasound.

**Clinical Pearl / High-Yield Fact**
The banana and lemon sign are crucial diagnostic features of spina bifida, a congenital anomaly that requires prompt identification and management to prevent long-term neurological sequelae.

**Correct Answer:** D