The retinoblastoma (RB) protein controls the G1 to S transition. The state of RB that enforces this cell cycle arrest, and the event that abolishes it, are respectively:

• A Hypophosphorylated RB binds E2F transcription factors; phosphorylation by cyclin D-CDK4/6 releases E2F ✓
• B Hyperphosphorylated RB binds E2F; dephosphorylation by PP2A releases E2F
• C Hypophosphorylated RB binds p53; acetylation of RB releases p53
• D Ubiquitinated RB binds E2F; deubiquitination by USP28 releases E2F

Explanation

In its active growth-suppressive form RB is hypophosphorylated and sequesters E2F, preventing transcription of S-phase genes such as cyclin E. Mitogenic signalling activates cyclin D-CDK4/6, which phosphorylates RB; hyperphosphorylated RB releases E2F and the cell enters S phase. Option B reverses the phosphorylation states, and options C and D incorrectly substitute p53 or ubiquitination as the binding partner.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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