The 'short-loop' negative feedback in the hypothalamo-pituitary axis refers to:

• A Anterior pituitary trophic hormone feeding back to inhibit the hypothalamus ✓
• B Peripheral target-organ hormone inhibiting hypothalamic releasing hormone secretion
• C Peripheral hormone feeding back to inhibit the anterior pituitary directly
• D Ultra-short loop where hypothalamic hormone inhibits its own release

Explanation

Feedback loops in the hypothalamo-pituitary axis are classified by loop length. The 'long loop' involves peripheral target hormones (e.g., cortisol) inhibiting both hypothalamus and pituitary. The 'short loop' refers to the anterior pituitary trophic hormone (e.g., ACTH, GH, LH) feeding back directly to the hypothalamus to inhibit releasing hormone secretion. The 'ultra-short loop' involves a hypothalamic hormone inhibiting its own release locally. Understanding loop hierarchy is important for interpreting pituitary function tests.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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