The PNP examines a 4-month-old infant with hypotonia and motor delay since birth. The most significant finding indicating lower motor neuron involvement in this infant would be:

The PNP examines a 4-month-old infant with hypotonia and motor delay since birth. The most significant finding indicating lower motor neuron involvement in this infant would be:

• A. Lack of deep tendon reflexes
• B. Hyperactive deep tendon reflexes
• C. Persistence of a Moro reflex
• D. Persistence of the tonic neck reflex

Answer: (A)

Lower motor neuron involvement shows up as reduced or absent reflexes because the reflex arc requires intact anterior horn cells and peripheral nerves. In an infant with hypotonia and motor delay since birth, not having deep tendon reflexes points to LMN pathology such as anterior horn cell disease or peripheral neuropathy, rather than central (upper motor neuron) problems. Upper motor neuron signals typically lead to increased reflex activity, like hyperreflexia, due to loss of inhibitory control. Persistence of primitive reflexes like Moro or tonic neck reflects delayed CNS maturation or central abnormalities, but does not specifically indicate LMN damage. So the absence of deep tendon reflexes is the most telling sign of lower motor neuron involvement.

Lower motor neuron involvement shows up as reduced or absent reflexes because the reflex arc requires intact anterior horn cells and peripheral nerves. In an infant with hypotonia and motor delay since birth, not having deep tendon reflexes points to LMN pathology such as anterior horn cell disease or peripheral neuropathy, rather than central (upper motor neuron) problems. Upper motor neuron signals typically lead to increased reflex activity, like hyperreflexia, due to loss of inhibitory control. Persistence of primitive reflexes like Moro or tonic neck reflects delayed CNS maturation or central abnormalities, but does not specifically indicate LMN damage. So the absence of deep tendon reflexes is the most telling sign of lower motor neuron involvement.