Oral Thrush (Candidiasis): This is the most common cause of a white plaque on the tongue that doesn't scrape off. It's a fungal infection, often seen in immunocompromised patients, those with poor oral hygiene, or after antibiotic use.

Leukoplakia: A condition where thick, white patches or lesions develop on the tongue, often associated with chronic irritation or tobacco use. It's a precursor to oral cancer, so it's crucial to monitor these lesions.
Lichen Planus: An autoimmune condition that can cause white, lacy patches on the tongue and inside the mouth. It's often associated with other symptoms like pain or burning sensations.
Oral Hairy Leukoplakia: Typically seen in immunocompromised patients, especially those with HIV/AIDS, this condition causes white patches on the tongue that cannot be scraped off.

Oral Cancer: Although less common, any persistent white plaque or lesion that doesn't scrape off could be a sign of oral cancer. Early detection is crucial for effective treatment.
Syphilis (Secondary): In its secondary stage, syphilis can cause a variety of oral manifestations, including white patches or lesions on the tongue. Given the potential for serious complications if left untreated, it's essential to consider syphilis in the differential diagnosis.

Pemphigus Vulgaris: A rare autoimmune disease that can cause blistering of the skin and mucous membranes, including the tongue, leading to white plaques or lesions.
White Sponge Nevus: A rare genetic condition that affects the mucous membranes, causing white, thickened patches on the tongue and sometimes other areas of the mouth. It's usually benign but can be a part of a syndrome with more significant health implications.