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Ketamine and esketamine matter because they act within hours rather than the weeks typical antidepressants take, offering a fast option for treatment-resistant depression (depression that has not responded to at least two adequate antidepressant trials). Both are NMDA (N-methyl-D-aspartate) receptor antagonistsA drug that blocks a receptor, stopping it from being activated.Read more →, meaning they block a glutamate receptor rather than working through serotonin or noradrenaline like most antidepressants. For example, imagine a patient with severe depression and active suicidal thoughts who cannot wait the usual four to six weeks for an SSRISelective serotonin reuptake inhibitor, a common type of antidepressant.Read more → to work: ketamine can lift mood within hours. This speed comes from a different, glutamate-based mechanism rather than the monoamine systems older antidepressants target.

By blocking NMDA glutamate receptors, ketamine triggers downstream changes that rapidly strengthen synaptic connections, producing an antidepressant effect within hours that can last days. Esketamine, the S-enantiomer of ketamine, is delivered as a nasal spray and is licensed specifically for treatment-resistant depression, given alongside an oral antidepressant under supervision. Both drugs commonly cause dissociationA disconnection between a person's thoughts, feelings, memories, or sense of identity, often as a response to stress.Read more → (a feeling of detachment from one's body or surroundings) during administration, which is why dosing happens in a monitored clinical setting. Say a patient feels detached and dreamlike for an hour after a dose: this is expected dissociation, not a sign of harm, and it resolves quickly. In practice, clinicians also watch for abuse potential and, with repeated recreational use, ketamine-induced cystitis (painful bladder inflammation).

• Blocking a glutamate receptor is only the first step in ketamine's antidepressant action. Which downstream process is most invoked to explain the sustained benefit?
• A man responds dramatically to a single ketamine infusion for treatment-resistant depression. What should he be told about how long that benefit typically lasts?
• A clinic protocol requires a specific physiological measurement before, during and after each esketamine dose. Which measurement is this, and why?
• A young man using ketamine recreationally several times weekly develops urinary frequency, urgency and suprapubic pain. Which complication does this most likely represent?

• SSRISelective serotonin reuptake inhibitor, a common type of antidepressant.Read more → (selective serotonin reuptake inhibitor): takes two to six weeks to work, unlike ketamine's antidepressant effect within hours.
• Phencyclidine (PCP), a related NMDA antagonistA drug that blocks a receptor, stopping it from being activated.Read more → misused recreationally: causes similar dissociationA disconnection between a person's thoughts, feelings, memories, or sense of identity, often as a response to stress.Read more →, whereas esketamine is delivered at a controlled, monitored medical dose.
• Electroconvulsive therapy (ECT), also effective in treatment-resistant depression, but works through induced seizures rather than glutamate receptor blockade.
• Recreational ketamine misuse, causes chronic cystitis (bladder inflammation) with repeated high-dose use, unlike the occasional monitored doses used clinically.

• The drug is not primarily a dopamine receptor blocker.
• Transporter changes describe conventional antidepressants instead.
• Benefit from a single dose rarely persists for months.
• Benefit outlasts the infusion by considerably longer than minutes.
• Durability is shorter, not longer, than conventional treatment.

• Identify the drug, receptor, enzyme or pharmacokinetic change named in the stem.
• Translate it into the immediate mechanism before looking at the options.
• Use timing and the adverse-effect pattern to separate the nearest alternatives.
• Check whether the question asks for mechanism, clinical consequence, monitoring or management.