Differential Diagnosis for Macular Rash, Fever, Headaches, Arthralgias, and Joint Swelling

Single Most Likely Diagnosis

• Viral Exanthem (e.g., measles, rubella, parvovirus B19): These viral infections commonly present with a macular rash, fever, and systemic symptoms such as headaches and arthralgias. The rash and fever often coincide with the onset of other symptoms.

Other Likely Diagnoses

• Lyme Disease: Caused by Borrelia burgdorferi, transmitted by tick bites. Early stages can present with a characteristic "bull's-eye" rash (erythema migrans), fever, headache, and joint pain.
• Rocky Mountain Spotted Fever (RMSF): A tick-borne illness caused by Rickettsia rickettsii, characterized by fever, headache, and a macular rash that may become petechial. Joint pain and swelling can also occur.
• Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), it can present with fever, sore throat, fatigue, and sometimes a rash, especially if the patient is taking ampicillin. Joint pain and swelling may also be present.

Do Not Miss Diagnoses

• Meningococcemia: A severe infection caused by Neisseria meningitidis, which can present with a petechial or macular rash, fever, headache, and joint pain. It is life-threatening and requires immediate treatment.
• Endocarditis: An infection of the heart valves, which can cause fever, joint pain, and sometimes a rash (e.g., Janeway lesions, Osler's nodes). Although less common, it is critical to diagnose due to its high morbidity and mortality.
• Disseminated Gonococcal Infection (DGI): A condition where Neisseria gonorrhoeae infects the bloodstream, causing fever, joint pain, and a rash. It is particularly important to consider in sexually active individuals.

Rare Diagnoses

• Southern Tick-Associated Rash Illness (STARI): Also known as Masters disease, it is characterized by a rash similar to erythema migrans but is transmitted by the lone star tick. It is less likely to cause systemic symptoms like fever and joint swelling compared to Lyme disease.
• Ehrlichiosis/Anaplasmosis: Tick-borne bacterial infections that can cause fever, headache, and sometimes a rash, along with joint pain. They are less common and might not always present with a rash.
• Secondary Syphilis: Can cause a wide range of symptoms including a macular rash, fever, and joint pain. The rash of secondary syphilis is often described as appearing on the palms and soles, which is distinctive.