This patient developed metabolic syndrome (weight gain, hyperglycemia) after being switched to a new antipsychotic, which is characteristic of atypical (second-generation) antipsychotics like olanzapine or clozapine. These agents exert their therapeutic effects and are also responsible for their metabolic side effect profile through combined antagonism of dopamine D2 and serotonin 5-HT2A receptors. Blockade of H1 and muscarinic receptors also contributes to weight gain and other side effects, but the core antipsychotic and metabolic mechanism involves D2 and 5-HT2A.

Phenelzine is a monoamine oxidase (MAO) inhibitor. MAO is responsible for breaking down dietary catecholamines like tyramine, which is found in aged cheeses, cured meats, and red wine. When MAO is inhibited, ingested tyramine is not metabolized, leading to a massive release of stored catecholamines (norepinephrine, epinephrine) from nerve terminals, causing a severe hypertensive crisis.

Lorazepam is a benzodiazepine. Benzodiazepines are positive allosteric modulators of the GABAA receptor. They bind to a site distinct from the GABA binding site and increase the frequency of chloride channel opening when GABA is bound. This enhances the inhibitory effect of GABA, leading to sedation, anxiolysis, and cognitive impairment, particularly in the elderly. Barbiturates increase the duration of channel opening.

Opioids like morphine cause constipation by activating μ-opioid receptors in the myenteric plexus of the gastrointestinal tract. This activation inhibits the release of acetylcholine from nerve endings, leading to decreased smooth muscle motility, reduced peristalsis, and increased colonic transit time, resulting in constipation.

Carbidopa is an inhibitor of peripheral aromatic L-amino acid decarboxylase (also known as DOPA decarboxylase). This enzyme converts levodopa to dopamine. By inhibiting this enzyme in the periphery, carbidopa prevents the conversion of levodopa to dopamine outside the central nervous system. This increases the amount of levodopa that can cross the blood-brain barrier and be converted to dopamine in the brain, while reducing peripheral side effects like nausea, vomiting, and arrhythmias caused by peripheral dopamine.

Gingival hyperplasia and hirsutism are classic side effects of phenytoin. Phenytoin is a primary antiepileptic drug that works by blocking voltage-gated sodium channels in their inactivated state. This action prolongs the refractory period of the neuron, preventing the rapid, repetitive firing that characterizes seizures.

Sertraline is a selective serotonin reuptake inhibitor (SSRI). SSRIs work by blocking the presynaptic serotonin transporter (SERT), leading to increased levels of serotonin in the synaptic cleft. Stimulation of various serotonin receptors, particularly 5-HT2 and 5-HT3, is responsible for the common side effect of sexual dysfunction, including decreased libido, delayed ejaculation, and anorgasmia.

The speed of induction and recovery for an inhaled anesthetic is inversely proportional to its solubility in blood, which is measured by the blood:gas partition coefficient. Anesthetics with a low blood:gas partition coefficient (like desflurane and nitrous oxide) are less soluble in blood. This means that the partial pressure of the gas in the alveoli equilibrates very quickly with the partial pressure in the blood and brain, leading to rapid induction and rapid elimination upon discontinuation.

Haloperidol is a high-potency typical (first-generation) antipsychotic that exerts its effects primarily through potent antagonism of dopamine D2 receptors. Blockade of D2 receptors in the mesolimbic pathway is therapeutic for positive psychotic symptoms. However, blockade of D2 receptors in the nigrostriatal pathway disrupts the normal balance of dopamine and acetylcholine, leading to an excess of cholinergic activity and producing extrapyramidal symptoms (EPS) such as parkinsonism (bradykinesia, rigidity, tremor) as seen in this patient.

Methylphenidate is a CNS stimulant used to treat ADHD. Its primary mechanism of action is the blockade of the dopamine transporter (DAT) and norepinephrine transporter (NET). This inhibition of reuptake increases the synaptic concentrations of dopamine and norepinephrine, particularly in the prefrontal cortex, which is thought to enhance executive function, attention, and impulse control.