Candidal Stomatitis: This is the most likely diagnosis given the suspicion. Candidal stomatitis, also known as oral thrush, is a common fungal infection in the mouth caused by Candida species, particularly Candida albicans. It presents with white patches on the mucous membranes of the mouth, which can be wiped off, leaving a red base that may bleed.

Angular Cheilitis: Often associated with Candida, this condition causes inflammation of the corners of the mouth and can be related to or coexist with candidal stomatitis.
Lichen Planus: An autoimmune condition that can affect the mucous membranes inside the mouth, presenting with white, lacy patches; red, inflamed tissues; or open sores.
Leukoplakia: A condition where thick, white patches or lesions develop on the tongue, the bottom of the mouth, or inside the cheeks, often linked to chronic irritation or tobacco use.

Oral Cancer: Although less likely, it's crucial not to miss oral cancer, which can present with ulcers, red or white patches, or other abnormalities in the mouth. Early detection is key to effective treatment.
Aphthous Ulcers: While typically benign, in immunocompromised patients, these ulcers can be a sign of an underlying condition that needs attention.
Histoplasmosis: A fungal infection that can cause oral lesions, particularly in immunocompromised individuals. It's essential to consider in patients with exposure history or systemic symptoms.

Pemphigus Vulgaris: A rare autoimmune disease causing blistering of the skin and mucous membranes, which can initially present in the mouth.
Erythema Multiforme: A skin condition that can also affect the mucous membranes, leading to oral lesions and ulcers, often in response to infections or medications.
Secondary Syphilis: Can cause a variety of oral manifestations, including mucous patches and ulcers, in patients infected with syphilis.