**Core Concept**
The asymmetric tonic neck reflex (ATNR) is a primitive reflex observed in infants, where flexion of one arm and extension of the opposite side occurs when the head is turned to one side. Its disappearance marks a key developmental milestone in motor system maturation, indicating the emergence of voluntary motor control.

**Why the Correct Answer is Right**
The ATNR typically appears in the first few months of life and is expected to disappear by **6 months of age**. This timing reflects the integration of the central nervous system, particularly the maturation of the cerebellum and cortical motor pathways. By 6 months, infants begin to exhibit voluntary control over movements, and the reflex is no longer present, signaling normal neurological development. Persistence beyond this age may suggest developmental delay or neurological disorders.

**Why Each Wrong Option is Incorrect**
Option B: 5 months – The reflex is still present at this stage; disappearance occurs after 6 months, not before.
Option C: 9 months – This is too late; by 9 months, most infants have already lost the reflex, and the reflex should be gone by 6 months.
Option D: 8 months – The reflex is no longer present by 6 months, so 8 months is beyond the expected window.

**Clinical Pearl / High-Yield Fact**
The disappearance of the ATNR by 6 months is a key indicator of normal neurological development. Persistence beyond 6 months may be a red flag for conditions like cerebral palsy or developmental delay and should prompt further neurological evaluation.

✓ Correct Answer: A. 6 months