Metastatic malignancy: This is often the most common cause of a unilateral liquified lymph node, as cancer cells can metastasize to lymph nodes and cause necrosis, leading to liquefaction. The most common primary sites include head and neck, breast, lung, and gastrointestinal tract cancers.

Tuberculosis (TB): TB can cause caseous necrosis in lymph nodes, leading to liquefaction. This is especially common in endemic areas or in immunocompromised patients.
Cat-scratch disease: Caused by Bartonella henselae, this disease can lead to lymphadenopathy with necrosis and liquefaction, typically after a cat scratch or bite.
Lymphoma: Certain types of lymphoma, such as Hodgkin lymphoma or non-Hodgkin lymphoma, can cause lymph node necrosis and liquefaction, especially if the lymphoma is aggressive or untreated.

Mycobacterial infections other than TB: Infections like leprosy or other non-tuberculous mycobacterial infections can cause similar symptoms and must be considered to avoid missing a treatable condition.
Kikuchi disease: A rare, self-limiting condition that can mimic lymphoma or TB, causing lymphadenopathy with necrosis.
Lymph node abscess: A bacterial infection of the lymph node can cause abscess formation and liquefaction, which requires prompt antibiotic treatment or drainage.

Kimura disease: A rare, chronic inflammatory disorder that can cause lymphadenopathy and eosinophilia, sometimes with necrosis and liquefaction of lymph nodes.
Castleman disease: A rare disorder that can cause lymphadenopathy, and in some cases, necrosis and liquefaction of lymph nodes, often associated with HIV infection or other immunocompromised states.
Rosai-Dorfman disease: A rare histiocytic disorder that can cause lymphadenopathy with necrosis and liquefaction, often accompanied by other systemic symptoms.