Pharmacology Questions (3) for USMLE, MCCEE, PLAB, FCPS, PDMC,MRCP………

A 23-year-old woman represents to her physician with ptosis, meiosis and anhidrosis in the left eye.

Which one of the following drugs is used to determine whether this lesion is preganglionic or postganglionic?







This is a typical case of Horner’s syndrome. Indirect sympathomimetic drugs such as cocaine act on postganglionic noradrenergic neurons. Administration of cocaine causes mydriasis if the lesion preganglionic but if it has no effect if the lesion is postganglionic. Direct sympathomimetic drugs such as epinephrine and phenylephrine acts on both pre- and postganglionic neurons. Beta-blockers have no effect on both neurons.

The correct answer is C

2 thoughts on “Pharmacology Questions (3) for USMLE, MCCEE, PLAB, FCPS, PDMC,MRCP………”

  1. cocaine (4%) dilates the normal pupil while as it has no effect on Horner pupil.Thus cocaine test diagnoses Horner’s syndrome.
    Amphetamine(1%) dilates normal pupil and in preganglionic and central Horner’s syndrome but not the post ganglionic Horner’s syndrome. Thus amphetamine test localises Horner’s syndrome as preganglionic/central.


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