Chronic Sphenoid Sinusitis: This is the most likely diagnosis given the patient's presentation of isolated sphenoid sinusitis with necrotic material, as it directly aligns with the symptoms and findings from the operation. The presence of necrotic material suggests a chronic infection.

Fungal Sphenoid Sinusitis: This could be a consideration, especially given the presence of necrotic material, which can be seen in invasive fungal infections. However, the lack of a history of diabetes or immunocompromised state makes this less likely but still a possibility.
Bacterial Sphenoid Sinusitis: This is another likely diagnosis, as bacterial infections are common causes of sinusitis. The presence of necrotic material could indicate a severe bacterial infection.

Mucormycosis: Although rare and more commonly associated with immunocompromised patients or those with diabetes, mucormycosis can present with necrotic tissue and is a critical diagnosis not to miss due to its high mortality rate if not promptly treated.
Invasive Aspergillosis: Similar to mucormycosis, invasive aspergillosis is more common in immunocompromised individuals but can be deadly if missed. It may present with similar symptoms, including necrotic material in the sinuses.

Sphenoid Sinus Cancer: Although extremely rare, cancer of the sphenoid sinus could present with symptoms of sinusitis and necrotic material. This would be an unusual diagnosis but should be considered in the differential to ensure no missed diagnoses.
Granulomatosis with Polyangiitis (GPA): Formerly known as Wegener's granulomatosis, GPA is a rare autoimmune disorder that can affect the sinuses and present with necrotic material due to granulomatous inflammation and vasculitis.
Tuberculous Sphenoid Sinusitis: Tuberculosis (TB) can rarely affect the sinuses, including the sphenoid sinus, and might present with chronic infection and necrotic material. This diagnosis is more likely in endemic areas or in individuals with a history of TB exposure.