Differential Diagnosis for the 8-year-old Patient
The patient presents with a rash that started as a small pimple-like lesion under the nose and spread to the upper extremities and chest/abdomen. Considering the history and symptoms, the differential diagnosis can be categorized as follows:
- Single most likely diagnosis
- Impetigo: Given the patient's history of exposure to impetigo a couple of months ago and the description of the rash starting as a small pimple-like lesion, impetigo is a strong consideration. The fact that the mother has been applying mupirocin, which is a common treatment for impetigo, but has seen no significant improvement, does not rule out this diagnosis, especially if the application has been sporadic.
- Other Likely diagnoses
- Contact Dermatitis: Although the father states they were not near poison ivy, the possibility of accidental exposure or exposure to another allergen or irritant cannot be entirely ruled out. The rash's spread and initial appearance could be consistent with contact dermatitis.
- Atopic Dermatitis (Eczema): The patient's history of applying triamcinolone (a topical steroid) suggests a consideration for atopic dermatitis, especially given the description of the rash and the mention of moistness on the left forearm. The patient's past medical history of anxiety and obsessive-compulsive disorder could also be relevant, as there is a known association between atopic dermatitis and psychological factors.
- Allergic Reaction: The recent discontinuation of Lamictal due to an allergic reaction raises the possibility of another allergic reaction, potentially to a different substance or medication.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Staphylococcal Scalded Skin Syndrome (SSSS): Although less likely, SSSS is a serious condition that can present with a rash and requires prompt diagnosis and treatment. The lack of significant improvement with mupirocin and the description of the rash could warrant consideration of this diagnosis.
- Steven-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Given the patient's recent history of an allergic reaction to Lamictal, SJS/TEN, although rare, must be considered, especially if there are signs of mucosal involvement or systemic symptoms, which are not explicitly mentioned but should be carefully evaluated.
- Rare diagnoses
- Pityriasis Rosea: This condition can present with a herald patch followed by a wider rash, but it is less likely given the initial description under the nose and the lack of characteristic distribution.
- Kawasaki Disease: Although very rare and typically presenting with fever, this condition can cause a rash and should be considered in the differential diagnosis of any child with a rash of unknown origin, especially if accompanied by other systemic symptoms.