Differential Diagnosis for the Seven-Year-Old Boy with a Rash

Single Most Likely Diagnosis

• Erythema Multiforme (EM): This condition is characterized by a symmetric macular rash with concentric circles, often appearing on the arms, hands, legs, and feet. The patient's history of a recent cold sore (herpes simplex infection) is a common trigger for EM, making it the most likely diagnosis.

Other Likely Diagnoses

• Urinary Tract Infection (UTI): Although less common in boys, a UTI could present with a rash, especially if it's a systemic infection. However, the lack of other symptoms such as dysuria, frequency, or abdominal pain makes this less likely.
• Viral Exanthem: Many viral infections can cause rashes, and given the patient's recent history of a cold sore and runny nose, another viral exanthem could be considered. However, the specific pattern of the rash and the lack of systemic symptoms make EM more likely.

Do Not Miss Diagnoses

• Meningococcemia: This is a life-threatening condition caused by Neisseria meningitidis, which can present with a rash. Although the rash in meningococcemia is typically petechial or purpuric, it's crucial to consider this diagnosis due to its high mortality rate if left untreated. The patient's lack of systemic symptoms such as fever, headache, or stiff neck makes this less likely, but it cannot be entirely ruled out without further evaluation.
• Rocky Mountain Spotted Fever (RMSF): This tick-borne illness can cause a rash, fever, and other systemic symptoms. Although the patient does not have a fever or other typical symptoms of RMSF, the rash can sometimes appear before the fever, making it a "do not miss" diagnosis in endemic areas.

Rare Diagnoses

• Kawasaki Disease: This condition is characterized by fever, rash, conjunctivitis, and other symptoms. Although the patient does not have a fever or other typical symptoms, Kawasaki disease can sometimes present atypically, especially in younger children.
• Lyme Disease: Caused by Borrelia burgdorferi, Lyme disease can present with a rash (erythema migrans), but it typically appears as a single lesion rather than a symmetric rash on multiple extremities. The lack of other symptoms such as arthritis or neurological findings makes this diagnosis less likely.