Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), this condition is common in teenagers and often presents with transient, multiple location lymphadenopathy, fever, and fatigue.

Viral Pharyngitis: Various viruses can cause pharyngitis, which may lead to lymphadenopathy, especially in the cervical region. The condition is usually self-limiting.
Streptococcal Pharyngitis: Although more commonly associated with acute symptoms, streptococcal infections can cause lymphadenopathy, particularly in the neck.
Reactive Lymphadenitis: This is a non-specific response to infection or inflammation elsewhere in the body, which can cause transient lymphadenopathy in multiple locations.

Hodgkin Lymphoma: Although less common, this type of cancer can present with lymphadenopathy that may initially seem transient or reactive. It's crucial to investigate further if lymphadenopathy persists or is accompanied by systemic symptoms like weight loss or night sweats.
Tuberculosis (TB): TB can cause lymphadenopathy, particularly in the cervical region, and is a critical diagnosis not to miss due to its public health implications and the need for specific treatment.
HIV Infection: Acute HIV infection can present with transient lymphadenopathy among other symptoms. Given the significant implications of this diagnosis, it should always be considered in the differential.

Kawasaki Disease: Although more commonly associated with younger children, Kawasaki disease can cause lymphadenopathy, particularly in the cervical region, along with other systemic symptoms like fever and rash.
Castleman Disease: A rare disorder that can cause lymphadenopathy, which may be localized or generalized. It's an important consideration in cases where lymphadenopathy is persistent or recurrent.
Lymphoma (Non-Hodgkin): Other types of lymphoma can also present with lymphadenopathy, though they are less common in teenagers compared to Hodgkin lymphoma.