Sharply demarcated erythematous plaque of the bilateral thighs.

Description of relevant history

A 52-year-old female with trisomy 21 and Alzheimer’s dementia presented with pruritic rash of the thighs 13 days after starting piperacillin-tazobactam and 3 days after starting ampicillin. The rash was initially on the left thigh, subsequently spreading to the right thigh. Physical examination demonstrated erythematous papules coalescing into a sharply demarcated plaque on the bilateral medial thighs with scattered nonfollicular pinpoint pustules and fine scaling (Fig 1). The patient was afebrile and without leukocytosis. Basic metabolic panel and complete blood count were within normal limits. Biopsy of the left medial thigh demonstrated interface dermatitis and subcorneal pustules (Fig 2).

Fig 1

Fig 2

Question 1: What is the most likely diagnosis?

Pustular contact dermatitis

Pustular psoriasis

Folliculitis

Acute localized exanthematous pustulosis (ALEP)

Dermatophytid infection

Answer:

Pustular contact dermatitis – Incorrect. Although the well-demarcated, bilateral nature of the patient’s rash may suggest pustular contact dermatitis, the histological findings are not consistent with this diagnosis.

Pustular psoriasis – Incorrect. The clinical course of pustular psoriasis is characterized by acute onset and recurrence. Additionally, patients with pustular psoriasis typically present with systemic findings such as fever and have a preceding diagnosis of psoriasis. This was the first episode in our patient, she was afebrile, and she did not have a preceding diagnosis of psoriasis, making this diagnosis less likely.

Folliculitis – Incorrect. Folliculitis is inflammation of the hair follicle, characterized by follicular-based pustules. The pustules were nonfollicular in this case.

ALEP – Correct. ALEP is a rare, localized variant of acute generalized exanthematous pustulosis. It is characterized by the rapid development of nonfollicular, sterile pustules on an erythematous base and is histologically characterized by subcorneal pustules, epidermal spongiosis, and perivascular infiltrate with neutrophils and eosinophils., ALEP is presumed to be caused by a T-cell–mediated type IV reaction.

Dermatophytid infection – Incorrect. Dermatophytid infections typically present as scaling papules and plaques with centrifugal spread. Progression of the infection results in central clearing. Although dermatophytid infections may present with pustules when follicular units are involved, the clinical course is inconsistent with this patient’s presentation.

Question 2: What is the most common etiology of this eruption?

Infection

Beta-lactam antibiotics

Herbal substances

Antiepileptic medications

Spider bites

Answer:

Infection – Incorrect. Infections are infrequently the cause of ALEP.,

Beta-lactam antibiotics – Correct. ALEP is most often drug-induced, and of those cases, beta-lactam antibiotics are the most common trigger.

Herbal substances – Incorrect. Although both topical and systemic exposure to herbal substances have induced ALEP, they are overall uncommon with only 8 cases published in the literature.

Antiepileptic medications – Incorrect. Antiepileptic medications have not been implicated in ALEP. It is a common trigger for drug reaction with eosinophilia and systemic symptoms.

Spider bites – Incorrect. Spider bites are an exceedingly rare cause of ALEP. Only 3 cases have reported spider bites as a cause of ALEP.

Question 3: Which of the following features of the patient’s presentation is most classic for this eruption?

Onset of rash after receiving piperacillin-tazobactam

Location on the extremities

Pruritus

Absence of fever

Leukocytosis

Answer:

Onset of rash after receiving piperacillin-tazobactam – Incorrect. ALEP tends to occur within 2 weeks of drug exposure and as quickly as 1 day with repeat medication exposure. The patient received piperacillin-tazobactam 13 days prior to the eruption’s onset, while ampicillin was administered 3 days prior. Therefore, the most probable culprit for the patient’s presentation is ampicillin.

Location on the extremities – Incorrect. ALEP has been reported to most frequently occur on the face, trunk, and upper limbs, in descending order.

Pruritus – Correct. Itching or burning sensation is a typical presenting feature of ALEP.

Absence of fever – Incorrect. Patients with ALEP may or may not present with fever. In a systematic review, 47.6% of patients with ALEP presented with fever.

Leukocytosis – Incorrect. Our patient presented without a leukocytosis, which is typical of ALEP as only 33.3% of patients may present with leukocytosis., Laboratory workup in ALEP is generally within normal limits.

Conflicts of interest

None disclosed.