**Core Concept**
Arnold-Chiari syndrome is a congenital condition characterized by the herniation of the cerebellar tonsils through the foramen magnum, often associated with meningomyeloencephalocele, and may involve other structural anomalies of the brain and spinal cord. This condition can lead to various complications, including hydrocephalus and neurological deficits.

**Why the Correct Answer is Right**
The correct answer, **C. Abnormal elongation of the medulla and lower cranial nerves**, is associated with Arnold-Chiari syndrome due to the impaired development of the neural tube during embryogenesis. The elongation of the medulla and lower cranial nerves is a direct consequence of the hindbrain herniation, which can lead to compression of these structures and subsequent neurological deficits. This phenomenon is a result of the abnormal formation of the posterior fossa and the resulting displacement of cerebellar and brainstem structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Fusion of the frontal lobes is not a characteristic of Arnold-Chiari syndrome. This condition primarily affects the posterior fossa and the brainstem, rather than the frontal lobes.

**Option B:** Fusion of the temporal, parietal, and occipital lobes is not a feature of Arnold-Chiari syndrome. While this condition may involve the cerebellum and brainstem, it does not typically affect the fusion of the cerebral lobes.

**Option D:** Partial or complete absence of the pituitary gland is not a characteristic of Arnold-Chiari syndrome. The pituitary gland is not directly involved in this condition, which primarily affects the posterior fossa and the brainstem.

**Clinical Pearl / High-Yield Fact**
Arnold-Chiari syndrome often presents with signs of cerebellar dysfunction, such as ataxia, dysphagia, and respiratory difficulties, due to the herniation of the cerebellar tonsils through the foramen magnum. Early recognition and management of this condition are crucial to prevent long-term neurological damage.

**✓ Correct Answer: C. Abnormal elongation of the medulla and lower cranial nerves**