Differential Diagnosis

• Single most likely diagnosis
  • Urticaria: The presentation of scattered blanching edematous wheals with central pallor, particularly on the trunk and proximal arms, is highly suggestive of urticaria. The fact that each lesion is less than 5 cm and some are coalescing further supports this diagnosis, as urticaria can present with varying sizes of wheals that may merge.
• Other Likely diagnoses
  • Acute allergic reaction (without anaphylaxis): Given the absence of hypotension, tongue/lip swelling, and other systemic symptoms, this could be considered a less severe allergic reaction. However, the lack of mucosal involvement and specific triggers makes urticaria a more straightforward diagnosis.
  • Insect bites or stings: These can cause similar skin reactions, but the scattered and coalescing nature of the lesions might suggest a more systemic or allergic cause rather than localized bites.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
  • Anaphylaxis: Although the question stem explicitly mentions the absence of key anaphylaxis symptoms (hypotension, mucosal erosions, tongue/lip swelling), it's crucial to consider anaphylaxis in any patient presenting with widespread urticarial lesions, as it can rapidly progress and become life-threatening.
  • Angioedema (if considering a variant without significant mucosal involvement): While the question specifies no tongue/lip swelling, some forms of angioedema can present without these classic symptoms. It's essential to monitor for any signs of airway compromise.
• Rare diagnoses
  • Erythema multiforme (EM) minor: Although target lesions are mentioned as absent, some cases of EM minor might not have the classic "target" appearance. However, the presence of mucosal involvement (absent in this case) and the specific pattern of the lesions would typically distinguish EM from urticaria.
  • Serum sickness: This condition can present with urticaria-like lesions but is often accompanied by other symptoms such as fever, arthritis, and sometimes renal involvement, which are not mentioned in the scenario.
  • Mastocytosis: A condition characterized by the accumulation of mast cells in one or more organs, which can lead to urticaria-like symptoms among others. However, it would be an unusual diagnosis to consider first without other supporting evidence or systemic symptoms.