What could cause pimple bumps on the thigh after sexual intercourse one week prior?

Pimple Bumps on Thigh After Sexual Contact

The most likely diagnoses are folliculitis from friction/irritation, contact dermatitis, genital warts (HPV), or herpes simplex virus, and you should examine the lesions for specific characteristics to guide management.

Initial Diagnostic Approach

The appearance and distribution of the lesions will determine your next steps:

Examine for Key Features

• If lesions are flesh-colored, dome-shaped papules: Consider genital warts caused by HPV types 6 or 11, which can occur on the thighs in addition to the genital area 1
• If lesions are grouped vesicles or painful ulcers: Consider genital herpes simplex virus, which can present on the thighs, particularly if there was direct skin-to-skin contact during sexual activity 2
• If lesions are open comedones (blackheads) or inflammatory papules: Consider acne mechanica from inner thigh friction during sexual activity 3
• If lesions are pruritic with visible parasites or nits: Consider pediculosis pubis, though this typically affects hair-bearing areas 1

Management Based on Lesion Type

For Suspected Genital Warts (HPV)

• Biopsy is rarely needed unless the diagnosis is uncertain, lesions don't respond to standard therapy, the disease worsens during therapy, the patient is immunocompromised, or warts are pigmented, indurated, fixed, and ulcerated 1
• Treatment options include patient-applied therapies (podofilox, imiquimod) or provider-administered therapies (cryotherapy, TCA/BCA, surgical removal) based on wart size, number, and patient preference 1
• No treatment eradicates HPV infection itself; the goal is removal of symptomatic warts 1

For Suspected Herpes Simplex Virus

• If painful perianal or genital ulcers are present, presumptive antiviral therapy for genital herpes should be initiated 1
• Obtain viral culture or PCR testing from vesicular fluid or ulcer base for confirmation 2

For Suspected Friction-Related Acne Mechanica

• This represents an acneiform eruption from friction, pressure, or rubbing during sexual activity 3
• Standard acne treatments (topical retinoids, benzoyl peroxide) are appropriate 3
• Advise on reducing friction during sexual activity

For Non-Specific Dermatitis

• If STI testing is negative and lesions are pruritic without specific infectious features, consider contact or irritant dermatitis 4
• Topical corticosteroids may provide symptomatic relief for inflammatory lesions 5

Critical Next Steps

• Test for concurrent STIs: Obtain syphilis serology and offer HIV testing with counseling for any patient presenting with genital lesions after sexual contact 1, 6
• Evaluate and treat sexual partners within the previous 60 days if an STI is diagnosed 1
• Instruct patients to abstain from sexual contact until evaluation is complete and any identified infection is adequately treated 1

Common Pitfalls to Avoid

• Don't assume benign lesions based on location alone—HPV warts can occur on the thighs, not just directly on genitalia 1
• Don't dismiss friction-related acne mechanica, which is a legitimate diagnosis in this context and represents the first reported bilateral case from inner thigh friction 3
• Don't forget that multiple HPV types can coexist, and visible warts don't predict cervical cancer risk 1
• If lesions persist or worsen after 2 weeks of empiric treatment, re-examination and possible biopsy are necessary 4, 5