The phototransduction cascade in rod photoreceptors involves a series of steps following photon absorption. The correct sequence after rhodopsin activation is:

• A R* activates Gs → adenylyl cyclase increases cAMP → PKA phosphorylates CNG channels causing them to close → hyperpolarization → decreased glutamate release
• B R* activates phospholipase C via Gq → IP3 releases Ca²⁺ from ER → Ca²⁺ closes CNG channels → depolarization → increased glutamate release
• C Activated rhodopsin (R*) activates transducin (Gt) → Gt activates PDE6 → cGMP hydrolysis → closure of CNG channels → hyperpolarization → decreased glutamate release at ribbon synapse ✓
• D R* directly phosphorylates phosphodiesterase 6 → reduced cGMP → CNG channel opening → Na⁺ influx → hyperpolarization via K⁺ efflux override

Explanation

Rod phototransduction: photon isomerizes 11-cis-retinal to all-trans-retinal → rhodopsin becomes activated (metarhodopsin II) → activates the trimeric G-protein transducin (Gαt) → Gαt binds and relieves inhibitory PDEγ subunit → activated PDE6 hydrolyzes cGMP → intracellular [cGMP] falls → cyclic nucleotide-gated (CNG) channels close → Na⁺ and Ca²⁺ entry stops → rod hyperpolarizes (from -40 mV to -70 mV) → reduced glutamate release at the ribbon synapse, signaling light detection to bipolar cells. Recovery involves guanylyl cyclase reactivation (by GCAP when Ca²⁺ falls) and rhodopsin kinase phosphorylation followed by arrestin binding.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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