**Core Concept**
The patient's symptoms suggest an orbital complication of sinusitis or an infectious process involving the cavernous sinus, given the location of pain and the signs of proptosis, chemosis, and diplopia. The involvement of the optic nerve and the cavernous sinus is a critical aspect of this presentation.

**Why the Correct Answer is Right**
The patient's symptoms are suggestive of a cavernous sinus thrombosis (CST), which is a serious and potentially life-threatening condition. CST typically occurs as a complication of orbital cellulitis or ethmoid sinusitis, which are common in young individuals who use nasal decongestants frequently. The clinical presentation of fever, chills, rigors, diplopia on lateral gaze, proptosis, chemosis, and congested optic disc is classic for CST. The involvement of the optic nerve and the cavernous sinus is a critical aspect of this presentation, which can lead to severe visual impairment if not treated promptly.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not consider the cavernous sinus involvement and the specific symptoms of diplopia on lateral gaze and congested optic disc.

**Option B:** This option is incorrect as it does not take into account the history of repeated pain over the medial canthus and the use of nasal decongestants, which are risk factors for sinusitis and CST.

**Option C:** This option is incorrect as it does not consider the specific symptoms of proptosis, chemosis, and congested optic disc, which are characteristic of CST.

**Clinical Pearl / High-Yield Fact**
Cavernous sinus thrombosis is a medical emergency that requires prompt recognition and treatment to prevent severe visual impairment and other complications. The key to diagnosing CST is to suspect it in any patient with symptoms of orbital or sinusitis complications, especially with a history of nasal decongestant use.

**Correct Answer:** C. Cavernous sinus thrombosis