<prompt> A 19 year old young girl with previous history of repeated pain over medial canthus and chronic use of nasal decongestants, presented with abrupt onset of fever with chills rigor, diplopia on lateral gaze, moderate proptosis chemosis. On examination optic disc is congested. Most likely diagnosis is -Cavernous sinus thrombosisOrbital cellulitisAcute Ethmoidal sinusitisOrbital apex syndrome  <response>: Repeated pain over medial canthus and chronic use of nasal decongestants suggest chronic ethmoidal sinusitis.Patients other symptoms suggest that she has landed up in complicated sinusitisNow lets see the symptoms one by one -There is sudden onset of -Chills rigor Systemic symptomsDiplopia on lateral gaze Suggestive of VI C.N. abducent involvement leading to lateral rectus palsyProptosis chemosisOptic disc congestionSo, among the given options -Orbital apex syndrome OAS can be ruled out as visual loss is often the initial manifestations of an OAS not seen in this patient.The remaining two options; cavernous sinus thrombosis orbital cellulitis can impose a diagnostic difficulty. Both of these have almost similar presentation with some differences.Abrupt onset of chills rigor, moderate proptosis and lateral gaze palsy favour cavernous sinus thrombosis CST. In orbital cellulitis onset is slow systemic features are mild there is restricted ocular movement in all directions from the beginning.Optic disc congestion vision loss in late stages are found both in CST orbital cellulitis. 