Infectious Mononucleosis (Mono): This is a common condition in children and adolescents, caused by the Epstein-Barr virus. The presence of a swollen and tender posterior cervical lymph node, along with pain in the left upper quadrant (LUQ) upon palpation (which could indicate splenomegaly, a common finding in mono), makes this a likely diagnosis.

Streptococcal Pharyngitis: Although the exam is otherwise normal, a throat infection could cause cervical lymphadenopathy. The pain in the LUQ could be unrelated or referred pain.
Viral Pharyngitis: Similar to streptococcal pharyngitis, viral infections can cause lymph node swelling and could be considered with a normal exam aside from the lymphadenopathy and LUQ pain.
Cat Scratch Disease: Caused by Bartonella henselae, this condition can lead to lymphadenopathy, typically in the region draining the site of the cat scratch, which could be the posterior cervical region if the scratch was on the head or neck.

Lymphoma: Although less common, lymphoma (such as Hodgkin lymphoma or non-Hodgkin lymphoma) can present with lymphadenopathy and must be considered to ensure timely diagnosis and treatment.
Tuberculosis (TB): TB can cause lymphadenitis, and although it's less common in some regions, it's crucial not to miss this diagnosis due to its public health implications and the need for specific treatment.
Leukemia: Certain types of leukemia can present with lymphadenopathy and abdominal pain due to splenomegaly or hepatomegaly, making it a critical diagnosis not to overlook.

Kawasaki Disease: An acute febrile illness that predominantly affects children, characterized by lymphadenopathy, among other symptoms. It's less likely given the specific location of the lymphadenopathy and the absence of other characteristic symptoms like rash, conjunctivitis, or changes to the lips and oral cavity.
Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can cause lymphadenopathy. It's much less common and would be considered if other diagnoses are ruled out.
Sarcoidosis: Although rare in children, sarcoidosis can cause lymphadenopathy. It would be an unusual diagnosis to consider in this age group without other systemic symptoms or findings.