An additional diagnosis that describes a condition arising after the beginning of hospital observation and treatment and then modifying the course of the patient's illness is called a

Complication

Explanation:

A complication in terms of medicine an be defined as the condition in which the there is an unfavorable result of the disease, treatment or health conditions.

It generally involves the worsening of the condition, disease, et cetera. The time span of the illness may increases after the patient is being admitted into the hospital.

this can be defined as the condition in which there is an increase in time in hospital with more complex diseases. new sign and symptoms may arise due to complications.