What is the most likely cause of a transient itchy erythematous rash on the neck and collar area of the forearms in a 55-year-old man with undetectable HIV?

Most Likely Cause: Contact Dermatitis or Drug Reaction

The most likely cause of a transient itchy red rash on the neck and collar aspect of forearms in a 55-year-old male with undetectable HIV is contact dermatitis (from clothing, detergents, or topical products) or a drug-induced reaction, rather than HIV-related dermatosis.

Clinical Reasoning

Why HIV-Related Causes Are Less Likely

The key clinical feature here is that the patient has undetectable HIV viral load. This fundamentally changes the differential diagnosis:

• HIV-associated pruritus typically correlates directly with viral load and occurs at late stages of infection 1
• With undetectable HIV, the patient is effectively immunologically stable and should not manifest HIV-specific dermatoses like eosinophilic folliculitis or pruritic papular eruption
• Pruritic papular eruption (PPE) serves as a marker for severe immunosuppression 2, which this patient does not have

Distribution Pattern Points to External Exposure

The specific anatomical distribution (neck and collar aspect of forearms) is highly suggestive of:

• Contact dermatitis from clothing collars, jewelry, watches, or topical products
• Areas of friction and contact with external materials
• This pattern is inconsistent with systemic HIV-related dermatoses, which typically present more diffusely

Drug-Induced Etiology Must Be Considered

Drug reactions are particularly common in HIV-positive patients due to immune dysregulation, altered drug metabolism, and polypharmacy 3. The guidelines emphasize obtaining a complete medication history including:

• All antiretroviral medications
• Over-the-counter pharmaceuticals
• Herbal remedies 1

Drug-induced pruritus can occur with or without rash, and the transient nature fits this diagnosis 1.

Diagnostic Approach

Essential History Elements

1. Detailed exposure history:

   • New clothing, detergents, fabric softeners, or jewelry
   • Occupational or recreational exposures
   • Recent changes in personal care products

2. Complete medication review:

   • Recent additions or changes to ART regimen
   • Any new medications (prescription or OTC)
   • Temporal relationship between medication changes and rash onset

3. Travel and environmental exposures 1

Physical Examination Focus

• Examine for distribution pattern consistent with contact exposure
• Look for vesicles, scaling, or lichenification suggesting allergic contact dermatitis
• Assess for signs of secondary infection from scratching

Management Strategy

First-Line Approach

1. Identify and remove the offending agent:

   • Discontinue suspected contactants
   • Review and potentially adjust medications if drug reaction suspected
   • Do not stop ART unless frank drug allergy is suspected 3

2. Symptomatic treatment:

   • Topical corticosteroids for localized contact dermatitis
   • Oral antihistamines for pruritus relief
   • Emollients for skin barrier restoration

When to Consider Alternative Diagnoses

Only if the rash persists despite removing obvious triggers should you consider:

• Skin biopsy to rule out other dermatoses
• Evaluation for systemic causes (though unlikely with undetectable viral load)
• Scabies (always consider in HIV patients with severe pruritus and minimal skin signs) 1

Critical Pitfalls to Avoid

• Do not assume all dermatologic issues in HIV patients are HIV-related - with undetectable viral load, common dermatoses are more likely
• Do not overlook medication history - drug reactions are more frequent in HIV-positive patients than immunocompetent individuals 3
• Do not discontinue ART prematurely - unless there is clear evidence of drug allergy 3
• Always look for a physical cause before labeling as psychogenic 1