Mononucleosis (Infectious Mononucleosis): This condition, caused by Epstein-Barr virus (EBV), is common in young adults and presents with fever, sore throat, and fatigue, often following a period of prodromal symptoms. The pattern of fever resolution followed by the return of fever with worsening symptoms like sore throat is consistent with the clinical progression of mononucleosis.

Streptococcal Pharyngitis: This bacterial infection can cause fever and severe sore throat. While it typically doesn't have a biphasic fever pattern, it's a common cause of sore throat in this age group and should be considered, especially if there's exposure to others with similar symptoms.
Influenza: Although influenza primarily presents with respiratory symptoms, it can also cause fever and sore throat. The return of fever could be part of the natural course of the illness or a complication like a secondary bacterial infection.
Viral Pharyngitis: Other viruses can cause similar symptoms, including fever and sore throat, and might have a variable course, including a temporary resolution of fever before worsening.

Lemierre's Syndrome: A rare but potentially life-threatening condition that starts with pharyngitis and can progress to septicemia and thrombophlebitis of the internal jugular vein. The initial presentation of sore throat and fever, followed by worsening symptoms, could be indicative of this condition.
Rheumatic Fever: Although less common in areas with access to antibiotics, rheumatic fever can follow untreated streptococcal pharyngitis and presents with fever, joint pain, and sometimes a migratory polyarthritis. The biphasic nature of the illness could fit this diagnosis if the initial fever was from the streptococcal infection and the second phase represents the onset of rheumatic fever.
Peritonsillar Abscess: A complication of tonsillitis that can cause severe sore throat, fever, and difficulty swallowing. It's a medical emergency requiring prompt drainage.

Toxoplasmosis: While more commonly associated with immunocompromised patients, toxoplasmosis can cause a mononucleosis-like syndrome in immunocompetent individuals, including fever and sore throat.
Cytomegalovirus (CMV) Infection: Similar to EBV, CMV can cause a mononucleosis syndrome, although it's less common and might be considered in the differential, especially if EBV is ruled out.
Human Immunodeficiency Virus (HIV) Acute Retroviral Syndrome: The initial presentation of HIV infection can mimic mononucleosis, with fever, sore throat, and fatigue, among other symptoms.