Infectious Mononucleosis (Mono): This condition, caused by Epstein-Barr virus (EBV), often presents with fever, body aches, and lymphadenopathy (which could explain the left cervical chain nodule). The red nodule to the left temporal area could be a manifestation of a rash or a specific type of skin lesion sometimes associated with mono.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with fever, night sweats, weight loss, and lymphadenopathy, which aligns with the symptoms of fever of unknown origin and the left cervical chain nodule.
Tuberculosis (TB): TB can cause fever, body aches, and lymphadenopathy. The red nodule could potentially be a cutaneous manifestation of TB, although this is less common.
Cat-Scratch Disease: Caused by Bartonella henselae, this infection can lead to lymphadenopathy (often in the neck), fever, and sometimes a papule or pustule at the site of the cat scratch, which could explain the red nodule.

Endocarditis: Although less likely, infective endocarditis is a serious condition that can present with fever of unknown origin, and it's crucial not to miss it due to its high mortality rate if left untreated. The presence of a new heart murmur or other cardiac signs would increase suspicion.
Neoplasm (e.g., Leukemia): Certain types of leukemia can present with fever, lymphadenopathy, and skin lesions. Given the severity of these conditions, they should always be considered in the differential diagnosis of fever of unknown origin.
Systemic Lupus Erythematosus (SLE): SLE is an autoimmune disease that can cause a wide range of symptoms, including fever, body aches, lymphadenopathy, and skin rashes. It's a diagnosis that should not be missed due to its potential for serious complications.

Kawasaki Disease: Although more common in children, Kawasaki disease can occur in adults and presents with fever, lymphadenopathy, rash, and sometimes desquamation in the hands and feet. The red nodule could be part of the rash associated with this condition.
Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by fever, neutrophilia, and tender erythematous skin lesions, Sweet syndrome is a rare condition that could potentially explain some of the symptoms, especially the red nodule and fever.
Erythema Induratum (Bazin Disease): A rare condition associated with TB, it causes recurrent erythematous subcutaneous nodules, usually on the calves but potentially elsewhere, including the temporal area.