Ptosis, loss of upward/medial movement and a lost light reflex on the left = complete left CN III palsy. There are no crossed long-tract signs, so the lesion is in the nerve itself (the sources list peripheral causes such as cavernous sinus or superior orbital fissure lesions), matching left orbit, oculomotor nerve.

• Midbrain lesions involving CN III in the sources come with contralateral hemiplegia or ataxia (Weber/Benedikt), which this patient lacks.
• MLF lesions cause internuclear ophthalmoplegia without ptosis or pupil involvement; the abducens nerve only affects abduction.