A 45-year-old man comes to the office for follow-up.  He was diagnosed with focal epilepsy 2 years ago and has been treated with several antiepileptic medications.  Over the last 6 months, his seizure frequency has increased despite compliance with medical therapy.  Neurologic examination and brain imaging are unremarkable.  The patient is started on a new antiepileptic medication that selectively blocks voltage-gated calcium channels.  This medication most likely affects which of the following steps of neurotransmission?

A 63-year-old man hospitalized for chronic obstructive pulmonary disease exacerbation develops hypercapnic respiratory failure.  He is transferred to the intensive care unit, where he undergoes intubation after premedication with muscle relaxant X and an appropriate sedative.  The patient is then started on mechanical ventilation; however, he remains apneic longer than expected.  Anesthesiology is consulted, and the patient’s neuromuscular blockade is assessed by electrically stimulating a peripheral nerve 4 times in quick succession and observing the muscular response.  The procedure is repeated 30 minutes later and the results are shown below.

Muscle relaxant X is most likely to be which of the following medications?

A 28-year-old woman with a history of type 1 diabetes mellitus comes to the hospital due to abdominal pain, nausea, and vomiting.  She is found to have severe hyperglycemia, high anion gap metabolic acidosis, and an elevated beta-hydroxybutyrate level.  While being evaluated in the emergency department, the patient develops involuntary, rhythmic jerking of her right upper extremity that lasts for about a minute.  She remains fully alert during and after the event.  The patient has never had similar symptoms in the past and has no other medical conditions.  Physical examination shows no weakness or sensory loss in the right upper extremity.  Which of the following is the most likely diagnosis?

A 64-year-old woman is brought to the clinic by her husband due to a fall. She was walking in her living room when she tripped over an electrical cord. The patient has had several other ground-level falls in the last 6 months but has had no significant injuries. She also gave up driving a year ago following a near-miss motor vehicle accident. Past medical history is notable for hypertension and type 2 diabetes mellitus, for which she takes appropriate medications. Mental status and neurologic examinations are normal. Ophthalmic examination reveals reduced vision in her peripheral visual fields bilaterally and an elevated intraocular pressure. After discussion of treatment options, the patient is initiated on timolol ophthalmic drops. Which of the following structures is the most likely target of this medication?

A 58-year-old airline captain comes to the office after he was found to have abnormal vision testing during a pilot medical certification examination.  The patient feels well and has no ocular symptoms.  Vision testing shows normal visual acuity but moderate loss of peripheral vision in both eyes.  Results of the funduscopic examination are shown in the image below.

Other neurologic and cardiopulmonary examination findings are unremarkable.  He is started on latanoprost eyedrops.  The medication is most likely to improve this patient’s condition through which of the following mechanisms?

A 48-year-old woman enrolls in a clinical trial.  The patient has had persistent discomfort in her lower extremities for the past 2 years.  She describes an uncomfortable sensation in her legs, particularly in the evening and at night, but no pain.  The symptoms are relieved with leg movement or walking.  She has experienced no behavioral or mood changes.  The patient intermittently experiences mild migraines, and during a recent episode she found that antiemetic therapy worsened the leg discomfort.  Vital signs are normal, and physical examination shows no abnormalities.  Blood cell counts, serum chemistry studies, and serum iron levels are within normal limits.  Treatment with a novel agent, an alpha-2-delta calcium channel ligand, is initiated.  The patient receives significant symptom relief with this therapy.  The novel agent used in this trial is most similar to which of the following medications?