June 2025 Case Study
Author: Alexis E. Carrington, MD
Reviewed by: Anna Yasmine Kirkorian, MD
A 5-month-old male with no significant medical history presents with a 3-month history of a generalized pruritic rash. The eruption began on the feet and spread upward. The patient’s mother reports significant sleep disruption due to itching. He failed oral valacyclovir and topical desonide previously prescribed. The patient’s mother and sister also developed pruritic rashes.
On examination, the patient had generalized inflammatory nodules and pustules, most prominent on the axillae, trunk, nape of the neck, and lateral feet.
Which of the following is the most appropriate initial diagnostic test for this patient?
Image 1: Many scattered erythematous-violaceous nodules extending from the left inner arm to the trunk.
Image 2: Several scattered pustules and inflammatory nodules on the lateral and dorsal left foot.
Image 3: Positive mineral oil prep demonstrating a live scabies mite and scybala.
A) Skin mineral oil preparation
B) Bacterial culture
C) Skin biopsy for histopathology
D) Fungal culture
Correct Answer: A) Skin mineral oil preparation
A mineral oil prep is the first-line, rapid, and inexpensive test to diagnose scabies by directly visualizing mites, eggs, or scybala under the microscope. Skin biopsy and cultures may be considered if the diagnosis remains uncertain after initial evaluation but are not the first step when scabies is strongly suspected based on clinical findings.
This patient has scabies confirmed by positive mineral oil prep. Scabies is caused by infestation with the mite Sarcoptes scabiei and presents with intensely itchy papules, nodules, and burrows, often worsening at night. Scabies is highly contagious via skin-to-skin contact and fomites.1,2 Scabetic nodules on the trunk, axillae and genitalia are the most common presentation in infants and can remain for several months after resolution.3
A skin mineral oil preparation (Choice A) is the most appropriate and efficient initial diagnostic test for confirming scabies by directly visualizing mites, eggs, or scybala under the microscope. A bacterial culture (Choice B) would only detect secondary bacterial infections, such as impetigo from scratching, and would not diagnose the underlying scabies infestation. A skin biopsy (Choice C) could potentially reveal mites or burrows under histopathology, but it is more invasive, time-consuming, costly, and less sensitive early in the disease. A fungal culture (Choice D) would be reasonable if considering deep fungal infections like coccidioidomycosis or cryptococcosis, but would be less likely to be seen in an immunocompetent patient.
All household members should be treated simultaneously to prevent reinfestation, and thorough cleaning of bedding, clothing, and fomites is crucial.
References:
- Bolognia, Jean L. Dermatology. 4th ed. Elsevier, 2018.
- James WD, Berger TG, et al. Andrews’ Diseases of the Skin: Clinical Dermatology. 13th ed. Elsevier, 2019.
- Paller AS, Mancini AJ. Cutaneous disorders of the newborn. In: Paller AS, Mancini AJ, eds. Hurwitz Clinical Pediatric Dermatology: A Textbook of Skin Disorders of Childhood & Adolescence. 6th ed. St. Louis, Missouri: Elsevier; 2021.
