Which condition is the MOST likely cause of an Argyll Robertson pupil?

Which condition is the MOST likely cause of an Argyll Robertson pupil?

• A. Tuberculosis
• B. Sarcoidosis
• C. Syphilis
• D. SLE

Answer: (C)

Light–near dissociation is the key idea here. An Argyll Robertson pupil constricts with accommodation (near response) but does not constrict to bright light. This pattern means the light reflex pathway is disrupted while the accommodation/convergence pathway remains intact, a mismatch that points to damage in the midbrain’s pretectal area rather than a primary iris or optic nerve issue. This specific sign is classically seen with neurosyphilis, especially in the late stages of syphilis that affect the dorsal midbrain (tabes dorsalis–related changes). The near response can still activate constriction through alternative neural routes, so the pupil can tighten when focusing on a near object despite a poor light response. Among the listed conditions, syphilis is the most likely cause of this exact pupil behavior. Tuberculosis, sarcoidosis, and SLE can involve the CNS but do not characteristically produce this light–near dissociation pattern.

Light–near dissociation is the key idea here. An Argyll Robertson pupil constricts with accommodation (near response) but does not constrict to bright light. This pattern means the light reflex pathway is disrupted while the accommodation/convergence pathway remains intact, a mismatch that points to damage in the midbrain’s pretectal area rather than a primary iris or optic nerve issue.

This specific sign is classically seen with neurosyphilis, especially in the late stages of syphilis that affect the dorsal midbrain (tabes dorsalis–related changes). The near response can still activate constriction through alternative neural routes, so the pupil can tighten when focusing on a near object despite a poor light response. Among the listed conditions, syphilis is the most likely cause of this exact pupil behavior. Tuberculosis, sarcoidosis, and SLE can involve the CNS but do not characteristically produce this light–near dissociation pattern.