Photo Quiz

Geometric Rash on the Leg


Am Fam Physician. 2018 Apr 15;97(8):531-532.

A healthy 62-year-old woman presented with a pruritic rash on her left ankle (Figure 1). The rash began five days earlier as numerous discrete, 2- to 3-mm erythematous papules, which coalesced into a large rectangular plaque. The patient noted the eruption three days after she applied a lidocaine patch to the area for pain. She had no history of similar symptoms. She had no history of allergy to latex, plastic, or other personal care products; atopic dermatitis; asthma; or allergic rhinitis. Patch testing was performed (Figure 2).

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Based on the patient's history and physical examination findings, which one of the following is the most likely diagnosis?

A. Allergic contact dermatitis.

B. Arthropod bite reaction.

C. Atopic dermatitis.

D. Irritant contact dermatitis.

E. Morbilliform drug eruption.


The answer is A: allergic contact dermatitis. The patient's history, the geometric nature of her eruption, and a positive patch test result are consistent with delayed T cell–mediated (type IV) hypersensitivity reaction to lidocaine. On the patch test, the patient had a reaction to lidocaine, the lidocaine patch, and the lidocaine injection (not shown in Figure 2). She was tested against all potential allergens contained in the patch (paraben preservative, adhesive, and plastic) but reacted only to lidocaine. Treatment of allergic contact dermatitis is symptomatic and usually includes a topical corticosteroid. The initial eruption typically subsides in three to four weeks.

Patch testing is used to diagnose type IV hypersensitivity reactions by exposing an area of skin to a suspected allergen for 48 hours. The skin is evaluated at 48 to 72 hours and again on day 5 to 7.1,2 A positive reaction may range from mild erythema and induration to severe vesiculation.3 Indications for patch testing include suspected allergic contact dermatitis; treatment-resistant chronic

Address correspondence to Marjorie E. Montanez-Wiscovich, MD, PhD, at Reprints are not available from the authors.

Author disclosure: No relevant financial affiliations.


show all references

1. Mowad CM, Anderson B, Scheinman P, Pootongkam S, Nedorost S, Brod B. Allergic contact dermatitis: patient management and education. J Am Acad Dermatol. 2016;74(6):1043–1054....

2. Pongpairoj K, Puangpet P, Thaiwat S, McFadden JP. Diagnosing allergic contact dermatitis through elimination, perception, detection and deduction. Am J Clin Dermatol. 2017;18(5):651–661.

3. Wentworth AB, Yiannias JA, Keeling JH, et al. Trends in patch-test results and allergen changes in the standard series: a Mayo Clinic 5-year retrospective review (January 1, 2006, to December 31, 2010). J Am Acad Dermatol. 2014;70(2):269–275.

4. Steen CJ, Carbonaro PA, Schwartz RA. Arthropods in dermatology. J Am Acad Dermatol. 2004;50(6):819–842.

5. Hamann CR, Hamann D, Egeberg A, Johansen JD, Silverberg J, Thyssen JP. Association between atopic dermatitis and contact sensitization: a systematic review and meta-analysis. J Am Acad Dermatol. 2017;77(1):70–78.

6. Ricketts EK, Willcox MJ, Steele RW. Fever and a morbilliform rash. Clin Pediatr (Phila). 2017;57(2):235–237.

This series is coordinated by John E. Delzell Jr., MD, MSPH, Associate Medical Editor.

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