Which sequence describes the HPA axis response to a perceived threat?

• A. Adrenal medulla releases adrenaline
• B. Pituitary releases ACTH followed by CRH from hypothalamus
• C. Thyroid hormones regulate stress
• D. Hypothalamus releases CRH, pituitary releases ACTH, adrenal cortex secretes cortisol

Answer: (D)

When a threat is perceived, the hypothalamus starts the HPA axis by releasing CRH. CRH then stimulates the anterior pituitary to release ACTH. ACTH travels to the adrenal cortex and triggers cortisol secretion. Cortisol helps mobilize energy by increasing glucose availability and modulating other body systems to cope with stress; it also feeds back to reduce CRH and ACTH levels to regulate the response. This sequence—CRH from the hypothalamus → ACTH from the pituitary → cortisol from the adrenal cortex—is the hallmark of the HPA axis response.

The other options mix in different parts of the stress response. Adrenaline release from the adrenal medulla is part of the faster sympathetic-adrenal system, not the HPA axis. The idea that thyroid hormones regulate stress doesn’t describe the immediate cascade of the HPA axis. And the sequence with CRH released after ACTH would not produce the correct pressure-and-pause pattern of the HPA response.