An 18-year-old man comes to the office due to a progressive rash. He first developed small, red spots on his feet 3 weeks ago. The rash has since spread to involve his scrotum, penis, lower abdomen, hands, and feet. The rash is extremely pruritic, and the patient is having trouble sleeping due to constant itching. He otherwise feels well. Vital signs are normal. Lesions on the feet are shown below.
Show Explanatory Sources
Which of the following additional information in this patient's history would best support the most likely diagnosis?
Scabies | |
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Clinical features |
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Treatment |
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The distribution and appearance of this patient's rash is characteristic of scabies. Scabies is due to an infestation by Sarcoptes scabiei, a mite that burrows into the epidermis and deposits eggs. Weeks after initial infection, patients develop a delayed type IV hypersensitivity reaction to the mite, mite feces, and mite eggs.
Presentation includes erythematous papules that are intensely pruritic, particularly at night. The classic distribution of lesions includes the extremities (eg, interdigital web spaces, flexor wrist surfaces, feet), axillae, umbilicus, and genitalia.
Scabies is highly contagious and spreads via direct person-to-person contact, and history often reveals a roommate or family member with a similar rash. Although less common, fomite transmission (eg, clothing) can also occur because mites survive 2-3 days off the skin.
(Choice A) Folliculitis from exposure to water contaminated with Pseudomonas aeruginosa presents as tender, pruritic papules or pustules on exposed areas, and symptoms typically self-resolve within 2 weeks. This patient's rash duration and distribution make this diagnosis unlikely.
(Choice B) Incomplete vaccination against varicella-zoster virus (VZV) increases the risk for chickenpox, which presents with fever (not seen here) followed by a pruritic, vesicular rash. Lesions are not common on the soles, and the rash typically crusts over and resolves in 1-2 weeks. VZV reactivation can present as herpes zoster, which may begin as erythematous papules but are typically painful and limited to a single dermatome.
(Choice C) Hiking in wooded areas increases the risk for poison ivy exposure or tick-borne diseases (eg, Lyme disease). Although contact dermatitis due to poison ivy causes an intensely pruritic rash, lesions are often linear (where the plant touched the skin) and on exposed surfaces. In addition, the rash typically resolves within 2-3 weeks. Lyme disease causes erythema migrans, an expanding macular rash not seen in this patient.
(Choice E) Syphilis, which causes a painless genital ulcer, spreads via sexual contact, and manifestations of secondary syphilis include a maculopapular rash involving the palms and soles. Unlike this case, however, most patients also have constitutional symptoms (eg, fever) and lymphadenopathy.
Educational objective:
Scabies causes an intensely pruritic, papular rash involving the hands, feet, and genitalia. Transmission is via direct person-to-person contact, and history often reveals a roommate or family member with a similar rash.