Differential Diagnosis for a 3-year-old with Low-grade Fever, Rash, Productive Cough, and Joint Pain

Single Most Likely Diagnosis

• Juvenile Idiopathic Arthritis (JIA): This condition is characterized by joint pain and swelling, which fits the bilateral foot and wrist pain described. The rash could be related to the systemic onset subtype of JIA, which can include a rash that varies in intensity. However, the presence of a productive cough might suggest an infectious component or another diagnosis.

Other Likely Diagnoses

• Viral Exanthem: Many viral infections can cause rashes, low-grade fevers, and systemic symptoms like joint pain in children. The rash that changes intensity could be consistent with certain viral exanthems.
• Infectious Arthritis: Bacterial or viral infections can cause arthritis, which would explain the joint pain. The rash and fever could be part of the infectious process.
• Kawasaki Disease: Although less common, Kawasaki disease can present with fever, rash, and joint pain. The productive cough might not be a typical initial symptom, but it's a diagnosis that needs consideration due to its potential for cardiac complications.

Do Not Miss Diagnoses

• Septic Arthritis: This is a medical emergency that requires prompt diagnosis and treatment. The symptoms of joint pain and fever could be indicative of septic arthritis, especially if the child shows signs of significant distress or if the joint pain is severe and localized.
• Meningococcemia: Although the rash description doesn't perfectly match the typical purpura of meningococcemia, any rash in the context of fever and systemic symptoms warrants consideration of this potentially life-threatening condition.
• Lyme Disease: If the child has been exposed to ticks, Lyme disease could be a consideration, as it can cause arthritis, rash (erythema migrans), and systemic symptoms.

Rare Diagnoses

• Rheumatic Fever: Following a streptococcal infection, rheumatic fever can cause arthritis, fever, and sometimes a rash. It's less common in very young children but should be considered, especially if there's a history of recent streptococcal infection.
• Systemic Lupus Erythematosus (SLE): Although rare in children as young as 3, SLE can cause a wide range of symptoms including joint pain, rash, and fever. It would be an unusual diagnosis at this age but could be considered if other diagnoses are ruled out and the child has a persistent or worsening course.