What initiates chemical thermogenesis in infants during cold stress?

The correct choice is the release of norepinephrine. During cold stress, an infant's body attempts to maintain its core temperature through a process called thermogenesis, which is critical in preventing hypothermia.

When an infant experiences a drop in body temperature, the sympathetic nervous system is activated, leading to the release of norepinephrine. This neurotransmitter plays a pivotal role in stimulating the metabolism of brown adipose tissue (brown fat) within the body. Brown fat is specialized for heat production in response to cold; it generates heat through a process known as non-shivering thermogenesis. Norepinephrine stimulates the breakdown of fatty acids stored in brown fat cells, leading to increased heat production without the muscular contractions seen in shivering.

Thus, the release of norepinephrine is the primary trigger that catalyzes this adaptive physiological response in infants, facilitating their ability to generate heat and maintain their body temperature during cold stress.