**Core Concept**
Inverse agonists are a type of ligand that bind to the same receptor as agonists but induce a pharmacological response opposite to that of the agonist. In the context of benzodiazepine receptors, inverse agonists would decrease the activity of the receptor, leading to a reduction in the anxiolytic effects of benzodiazepines.

**Why the Correct Answer is Right**
Flumazenil is a selective inverse agonist of the benzodiazepine receptor. It acts by binding to the GABA_A receptor complex, which is composed of several subunits including the benzodiazepine receptor. By binding to this receptor, flumazenil reduces the frequency of chloride channel opening, thereby decreasing the inhibitory effect of GABA on neuronal excitability. This results in increased anxiety and seizure activity, opposite to the effects of benzodiazepines.

**Why Each Wrong Option is Incorrect**
**Option A:** Midazolam is a benzodiazepine agonist, not an inverse agonist. It acts by increasing the frequency of chloride channel opening, leading to sedation and anxiolysis.
**Option B:** Picrotoxin is a GABA antagonist, not an inverse agonist. It acts by blocking the GABA receptor, leading to increased neuronal excitability and seizures.
**Option C:** Phenobarbital is a barbiturate agonist, not an inverse agonist. It acts by increasing the frequency of chloride channel opening, leading to sedation and anxiolysis.

**Clinical Pearl / High-Yield Fact**
Flumazenil is used clinically to reverse the effects of benzodiazepine overdose, but it can also precipitate seizures in patients with a history of seizure disorders. This highlights the importance of carefully evaluating patients before administering flumazenil.

**Correct Answer: D. Flumazenil**