**Core Concept**
The tonic neck reflex is a primitive reflex present in infants, where flexion of one arm and extension of the opposite side occurs when the head is turned. Its disappearance is a developmental milestone, typically occurring by 4–6 months of age, and is associated with the maturation of the central nervous system, particularly the **corticospinal tracts** and **brainstem integration**.

**Why the Correct Answer is Right**
The tonic neck reflex is mediated by the **brainstem**, specifically the **medulla oblongata** and upper brainstem structures. As the infant develops, the reflex is suppressed by higher cortical control and descending pathways. The **medulla** integrates reflex arcs and contains the **reticulospinal tracts** that modulate spinal reflexes. A lesion in the medulla disrupts this integration, leading to persistent or abnormal reflex activity, and failure to lose the tonic neck reflex. This is especially evident in conditions like **cerebellar or brainstem lesions**.

**Why Each Wrong Option is Incorrect**
Option A: The cerebral cortex is involved in **suppression** of primitive reflexes, not their origin. A lesion here may delay loss of the reflex, but it does not cause its persistence or loss due to a lesion.
Option B: The midbrain contains the **superior and inferior colliculi** and is involved in motor control, but not the primary site of tonic neck reflex integration.
Option D: Spinal cord lesions may affect reflexes, but the tonic neck reflex is a **brainstem-mediated** reflex and is not primarily governed by spinal cord circuits.

**Clinical Pearl / High-Yield Fact**
Loss of primitive reflexes like tonic neck reflex is a key indicator of **neurological maturity**; their persistence beyond 6 months may suggest **brainstem or cerebellar pathology** and is a red flag in pediatric neurology.

✓ Correct Answer: C. Medulla