Differential Diagnosis for a 10-year-old Boy with a Skin Rash

The patient's presentation of a skin rash that started with a single red spot over a small scratch and spread over 4 days, along with swollen lymph nodes, suggests an infectious or allergic etiology. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis
  • Impetigo: A highly contagious bacterial skin infection that often starts as a single red spot and spreads, commonly seen in children. The presence of a scratch as the initial site and the spread of lesions are consistent with impetigo, which is typically caused by Staphylococcus aureus or Streptococcus pyogenes.
• Other Likely Diagnoses
  • Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can start at a site of trauma, such as a scratch. It presents with redness, swelling, and warmth, and can have systemic symptoms like fever, though the patient's temperature is normal.
  • Allergic Contact Dermatitis: An allergic reaction to something that has come into contact with the skin, which could start at the site of a scratch if the allergen entered through the wound. This condition can cause a rash, itching, and swelling.
• Do Not Miss Diagnoses
  • Rabies: Although less likely, if the scratch was from an animal, rabies must be considered due to its severe consequences if not treated promptly. Early symptoms can include a rash or lesion at the site of the bite.
  • Cat Scratch Disease (Bartonellosis): Caused by Bartonella henselae, transmitted through cat scratches. It leads to swollen lymph nodes, which are present in this case, and can cause a rash or papule at the site of the scratch.
• Rare Diagnoses
  • Erythema Multiforme: An immune-mediated disorder that can be triggered by infections or medications, characterized by target-like lesions on the skin. It's less common in children and typically has a distinct appearance.
  • Lyme Disease: Caused by Borrelia burgdorferi, transmitted by tick bites. Early stages can include a distinctive "bull's-eye" rash (erythema migrans) at the site of the bite, though this is not explicitly mentioned.

Given the information, the most appropriate initial treatment would likely involve antibiotics (Option C) for a presumed bacterial infection such as impetigo, considering the description of the rash starting from a scratch and spreading. However, the final choice of treatment should be guided by a definitive diagnosis, which may require further investigation, including cultures or other diagnostic tests.