Behçet's Disease: This condition is characterized by recurrent mouth ulcers, genital ulcers, and uveitis, and it can also cause aseptic meningitis. The combination of mouth ulcers and aseptic meningitis makes Behçet's Disease a strong candidate for the diagnosis.

Herpes Simplex Virus (HSV) Infection: HSV can cause both mouth ulcers (herpes labialis) and aseptic meningitis, especially in cases of primary infection. The presence of these two symptoms could suggest an HSV infection.
Enteroviral Infection: Certain enteroviruses can cause aseptic meningitis and may also be associated with mouth ulcers, although this is less common.
Systemic Lupus Erythematosus (SLE): SLE is an autoimmune disease that can cause a wide range of symptoms, including mouth ulcers and aseptic meningitis, particularly in patients with active disease.

Multiple Sclerosis: Although less likely, multiple sclerosis can present with aseptic meningitis and, in rare cases, oral ulcers. Missing this diagnosis could lead to delayed treatment and significant morbidity.
Sjögren's Syndrome: This autoimmune disorder primarily affects the exocrine glands, causing dry mouth and eyes, but it can also lead to aseptic meningitis and oral ulcers in some cases.
Vasculitis (e.g., Giant Cell Arteritis, Wegener's Granulomatosis): Various forms of vasculitis can cause aseptic meningitis and oral ulcers, and missing these diagnoses could result in severe complications.

Kawasaki Disease: Primarily seen in children, Kawasaki disease can cause mouth ulcers and, in rare instances, aseptic meningitis. It's essential to consider this in pediatric patients with these symptoms.
Erythema Multiforme: This skin condition can cause mouth ulcers and, in severe cases (Stevens-Johnson syndrome), may be associated with aseptic meningitis, although this is rare.
Sarcoidosis: Sarcoidosis is a systemic disease that can affect multiple organs and cause aseptic meningitis and oral ulcers, although this combination is uncommon.