What is the most likely diagnosis for a 6-year-old boy presenting with fever, pruritic rash, and joint pain, who was recently diagnosed with streptococcal pharyngitis and is on oral Penicillin (penicillin), with symptoms including hyperthermia, a diffuse rash on the arms and back, tender wrists and ankles, and palpable lymph nodes in the cervical, axillary, and inguinal regions?

Differential Diagnosis

• Single most likely diagnosis
  • E. Serum sickness–like reaction: This condition is a common adverse reaction to certain medications, including penicillin. The patient's symptoms, such as fever, pruritic rash, and joint pain, are consistent with a serum sickness-like reaction, which typically occurs 7-10 days after initiation of the offending medication. The patient's recent diagnosis with streptococcal pharyngitis and treatment with oral penicillin support this diagnosis.
• Other Likely diagnoses
  • A. Acute rheumatic fever: Although the patient has a history of streptococcal pharyngitis, the lack of migratory polyarthritis, carditis, and other major Jones criteria makes this diagnosis less likely. However, the joint pain and fever could still be consistent with acute rheumatic fever.
  • C. Henoch-Schönlein purpura: This condition typically presents with palpable purpura, abdominal pain, and arthritis. While the patient has joint pain and palpable lymph nodes, the absence of purpura and abdominal pain makes this diagnosis less likely.
• Do Not Miss
  • B. Anaphylaxis: Although anaphylaxis is a life-threatening condition that typically presents with rapid onset of symptoms, including rash, hypotension, and respiratory distress, it is essential to consider this diagnosis due to the patient's recent medication use and potential for severe allergic reactions.
  • F. Stevens-Johnson syndrome: This rare but severe condition is characterized by skin and mucous membrane lesions, often accompanied by fever and systemic symptoms. While the patient's symptoms do not entirely align with Stevens-Johnson syndrome, it is crucial to consider this diagnosis due to its potential severity and the patient's recent medication use.
• Rare diagnoses
  • D. Scarlet fever: This condition typically presents with a distinctive rash, fever, and strawberry tongue, often accompanied by streptococcal pharyngitis. However, the patient's symptoms and physical examination do not entirely align with scarlet fever, making it a less likely diagnosis.