Reactive lymphadenitis: This is the most common cause of lymphadenopathy in adults, often resulting from a viral or bacterial infection, such as a cold, flu, or dental infection. The location in the mandibular region suggests a possible dental or oral source of infection.

Dental infection or abscess: Infections of the teeth or gums can cause lymphadenopathy in the mandibular region due to the direct drainage of lymph from these areas.
Tuberculosis (TB): Although less common in some regions, TB can cause lymphadenopathy, including in the mandibular area, especially in individuals who have been exposed or have a weakened immune system.
Toxoplasmosis: This parasitic infection can cause lymphadenopathy, often in the cervical region, which includes the mandibular area, particularly in immunocompromised individuals.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with lymphadenopathy. Although less common, missing this diagnosis could have severe consequences, making it crucial to consider, especially with persistent or worsening symptoms.
Metastatic cancer: Cancers of the head and neck, such as squamous cell carcinoma, can metastasize to lymph nodes in the mandibular region. Early detection is critical for treatment outcomes.
HIV infection: Acute HIV infection can cause lymphadenopathy, and the mandibular region can be involved. Given the significant implications for patient management and public health, this diagnosis should not be overlooked.

Sarcoidosis: This autoimmune disease can cause lymphadenopathy, including in the mandibular region, although it is less common.
Cat-scratch disease: Caused by Bartonella henselae, this infection can lead to lymphadenopathy, typically after a cat scratch or bite, and can occasionally involve the mandibular lymph nodes.
Kikuchi-Fujimoto disease: A rare, self-limiting condition that can cause lymphadenopathy, often in the cervical region, including the mandibular area, more commonly in young women.