Final answer:
Phenobarbitone enhances the opening of the chloride channel upon activation of GABA A receptors, not calcium channels, and is not effective for absence seizures. It has a pKa of about 7.2.
Step-by-step explanation:
Regarding Phenobarbitone, it is incorrect to say that it enhances GABA mediated opening of the Ca channel. Instead, Phenobarbitone, which is a type of barbiturate, enhances the activity of the GABA receptor, but it primarily increases the duration of the opening of the chloride (Cl-) channel, not calcium channels. By promoting chloride ion influx, it hyperpolarizes the neuron, moving it further from the threshold of excitation, and thus has a sedative effect on the brain. As for absence seizure activity, Phenobarbitone is not effective as an absence seizure medication; it is used for different types of seizures. Furthermore, the pKa of Phenobarbitone is approximately 7.2, which is the only correct option provided.