Calcitriol regulates calcium homeostasis by altering gene transcription in intestinal epithelial cells. Its activated receptor complex exerts this effect by:

• A Binding DNA as a homodimer at direct repeat response elements
• B Anchoring ribosomes to the endoplasmic reticulum to boost translation
• C Phosphorylating histone H1 through intrinsic kinase activity
• D Binding DNA as a heterodimer with the retinoid X receptor ✓

Explanation

The vitamin B receptor is a nuclear receptor that must dimerize with the retinoid X receptor (RXR) before binding vitamin B response elements and recruiting coactivators. Glucocorticoid and mineralocorticoid receptors, by contrast, bind as homodimers on palindromic elements. Nuclear receptors possess no kinase activity, eliminating option C.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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