Which sign is indicative of neurological involvement in an infant?

The presence of decreased tone in an infant is indicative of neurological involvement because it can signify an impairment in the central nervous system's capacity to maintain normal muscle tone. In infants, muscle tone is crucial for motor control and coordination. When there is diminished tone, it often points towards neurological issues, such as conditions affecting the brain or spinal cord.

Normal fontanelle size typically suggests that there are no significant issues with intracranial pressure or developmental concerns, while clear mucosa indicates that there are no upper respiratory issues present. Similarly, normal vital signs are generally a sign of stable systemic function and typically do not point to neurological anomalies. Hence, while these factors contribute to an overall assessment of an infant's health, decreased tone specifically highlights the possibility of central nervous system involvement, making it the key sign to note in this context.