Final answer:
A nurse should intervene if a patient shows signs of gangrenous inflammation, infection, or sensory issues such as pins and needles, as these can be serious complications of prolonged immobility.
Step-by-step explanation:
A sign that would indicate a nurse should intervene if a patient has been immobile for the last three weeks could be evidence of gangrenous inflammation and infection. This is a serious condition where prolonged immobility can lead to tissue death and infection which can spread if not promptly treated. Due to the infrequent changing of clothes and bedding, these signs might be missed, which is why it's important for healthcare providers to perform regular skin checks and hygiene interventions.
Furthermore, loss of sensation or a presence of pins and needles, as described by the patient when touched, indicates a potential nervous system issue necessitating careful evaluation and possible intervention, such as setting up a consult for diagnostics or starting therapies to prevent further complications, like the mentioned aspirin therapy to limit blood clots. Regular assessments for sensory and muscular function should be conducted to detect any changes that require immediate care.