Short answer

Any new genital sore deserves prompt medical evaluation. Same-day to within days, before healing erases diagnostic yield.

STI causes include herpes (painful blisters), syphilis (classically painless chancre), chancroid, and LGV. But trauma, folliculitis, yeast, eczema, Behçet’s disease, and fixed drug eruptions mimic them closely.

Appearance never diagnoses: painful-versus-painless patterns hint, but real sores break textbook rules constantly, and only swab NAAT plus blood tests distinguish causes.

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STI causes at a glance. Patterns with their tests

  • Herpes: grouped blisters rupturing into tender ulcers, often with burning urination or flu-like first episodes (fresh sore swabs with HSV typing. Yield highest on unroofed vesicles).
  • Syphilis: single painless ulcer with firm edges healing spontaneously within weeks (healing never equals cured. Infection progresses silently; serology stages and confirms).
  • Chancroid (rare in the US): soft painful sores with tender bubo lymph nodes.
  • LGV: small transient sore followed weeks later by painful groin swelling with groove sign.

Each has distinct testing, sore swabs, blood tests, node examination, ordered together where patterns overlap.

Evaluation detail at Genital Sores: Causes and Evaluation.

Non-STI look-alikes. The common confusers

  • Ingrown hairs and folliculitis (hair-bearing areas, shaving-associated, pustular centers).
  • Friction tears (mechanical contexts, linear patterns).
  • Contact irritation (new soaps, latex, lubricants).
  • Severe yeast (satellite pustules with intense itching).
  • Psoriasis and eczema (extragenital disease clues).
  • Autoimmune ulcers (Behçet’s oral-genital-eye triads, Crohn’s fissured patterns).

Assuming an STI, or assuming “just irritation”, both delay correct care equally; the workup sorts them the same day either way.

Testing path. Speed decides yield

  • Clinicians swab fresh sores for herpes NAAT (present with unhealed lesions. Do not apply creams beforehand, do not wait for “a better time”).
  • Syphilis blood tests drawn regardless of suspected cause (syphilis mimics everything, and coinfection is common).
  • Lymph-node and full genital examination (nodes, discharge, rash. The company localizing the cause).

Because sores heal daily, earlier evaluation tests more accurately. Day-two visits outperform day-twelve visits with identical thoroughness.

While you wait. Protect the diagnostic window and partners

  • Abstain from all sexual activity including oral sex (sores shed regardless of barriers).
  • Keep the area clean and dry (water only. No picking, no harsh antiseptics, no potent steroids obscuring morphology).
  • List recent partners and symptom dates (timelines diagnosing. Written before anxiety edits them).
  • Prepare partner-notification drafts (exposure disclosure now, specifics after results).

When to seek care

Promptly for any new sore; urgently for rapidly spreading pain, swelling, fever, inability to urinate, or sores in pregnancy.