Final answer:
The major stressor of hospitalization for children from middle infancy through the preschool years is separation anxiety, due to their developmental stage where they recognize the caregiver's absence but cannot understand its reasons.
Step-by-step explanation:
The major stressor of hospitalization for children from middle infancy through the preschool years is separation anxiety. During middle infancy, when object permanence develops, children also begin to exhibit stranger anxiety. They might feel distressed when caregivers leave them, even in a hospital setting, which they cannot fully comprehend. Separation anxiety occurs because children recognize their caregiver's absence but cannot understand why it is happening. Preschool children, while beginning to assert control in familiar settings, may still experience intense separation anxiety in unfamiliar environments such as hospitals.