Differential Diagnosis

The patient presents with two distinct complaints: an itchy rash and chronic right shoulder pain post-surgery. The differential diagnoses for these complaints can be categorized as follows:

• Single Most Likely Diagnosis

  • Contact dermatitis or allergic reaction for the itchy rash, given the onset after eating pizza and the presence of a maculopapular rash with excoriation marks. The use of cetirizine with little effect suggests a possible allergic component.
  • Adhesive capsulitis or frozen shoulder for the chronic right shoulder pain, considering the severely limited range of motion and tenderness on palpation, which could be a complication of the shoulder replacement surgery.

• Other Likely Diagnoses

  • Eczema (atopic dermatitis) for the itchy rash, despite no previous history, as it can present at any age and the description of intense pruritus, especially at night, is consistent with eczema.
  • Irritant contact dermatitis, given the widespread nature of the rash and the lack of response to cetirizine, suggesting a possible irritant cause rather than an allergic one.
  • Osteoarthritis or hardware failure for the shoulder pain, as potential complications of the shoulder replacement surgery, though less likely given the description of severe pain and restricted movement without mention of trauma or infection signs.

• Do Not Miss Diagnoses

  • Infection (e.g., cellulitis, abscess): Although the patient does not have fever, redness, or skin changes typical of infection, it's crucial to consider, especially given the history of surgery. Infections can present subtly, especially in the context of immunocompromised states or certain types of bacteria.
  • Lymphoma or other malignancies: Though rare, certain malignancies can present with itching without a clear rash or with a maculopapular rash. Given the absence of other symptoms and the acute onset, this is less likely but should be considered if the rash does not respond to treatment.
  • Thyroid disorders: Both hyperthyroidism and hypothyroidism can cause skin changes and itching. Given the widespread nature of the rash and the absence of other systemic symptoms, this is less likely but should be considered in the differential.

• Rare Diagnoses

  • Pityriasis rosea: Characterized by a herald patch followed by a widespread rash, it could be considered given the description of the rash, though the intense pruritus and the onset after eating pizza make it less likely.
  • Dermatitis herpetiformis: An autoimmune blistering disorder associated with celiac disease, characterized by intensely itchy blisters. The absence of blisters and the description of the rash make this less likely.
  • Sjögren's syndrome: An autoimmune disorder that can cause dry skin and itching among other symptoms. Given the acute onset and the absence of other symptoms like dry eyes or mouth, this is unlikely but could be considered if other diagnoses are ruled out.