Final answer:
A patient with an incapacitating systemic disease that constantly threatens life is classified as P4 in the ASA Physical Status Classification System.
Step-by-step explanation:
According to the criteria established by the American Society of Anesthesiologists (ASA), a patient with an incapacitating systemic disease that is a constant threat to life would be categorized in the ASA Physical Status Classification System as class P4. This classification is part of an assessment scale used to evaluate a pre-anesthetic medical comorbidities risk.
The case mentioned involving diffuse gangrenous inflammation and infection, where the patient cannot move and faces a life-threatening condition fits into the P4 category. The P4 class specifically indicates a patient with severe systemic disease that is a constant threat to life, which aligns with the condition of a patient suffering from extensive gangrene.