12-phase flow (12)

1. 1FRAME
   Frame PR as a common, self-limited, presumed-viral (HHV-6/7) papulosquamous eruption — the dominant clinical task is CONFIDENT DIAGNOSIS + RULING OUT SECONDARY SYPHILIS, not pharmacotherapy. Most cases resolve in 6-8 weeks with reassurance + symptomatic care. Atypical, palm/sole, mucosal, recurrent, or pregnancy-onset cases need active workup. The not-to-miss is secondary syphilis masquerading as PR.
   advance: PR vs syphilis vs drug-eruption framing set; pregnancy + duration thresholds noted
2. 2ENTRY
   Recognise the classic course: herald patch -> generalised oval scaly eruption with collarette in Christmas-tree distribution on the trunk; or recognise an atypical/pregnant/drug-linked/sexually-active variant that needs active workup. Capture morphology + collarette scale up front.
   inputs: lesion_morphology_with_collarette
   actions: workup.chronic_pruritus
   advance: entry trigger recognised; morphologic features recorded
3. 3CONTEXT
   Build the differential context: distribution (trunk Christmas-tree vs palm/sole + mucosa), herald-patch history, sexually-active + STI risk factors, recent drug initiation or vaccination (the pityriasiform DRUG eruption + post-vaccine pattern), pregnancy + gestational age, prior similar episode (recurrence), and constitutional symptom burden.
   inputs: distribution_pattern, herald_patch_history, sexually_active_with_risk_factors, recent_drug_or_vaccine_exposure, pregnancy_status_and_gestational_age
   actions: workup.chronic_pruritus
   advance: differential context complete; pregnancy + drug-link + STI-risk states recorded
4. 4RED_FLAGS
   PALM / SOLE involvement, MUCOSAL patches (mouth, genital), CONDYLOMA LATA, generalised lymphadenopathy, alopecia areolaris — these signal SECONDARY SYPHILIS, not PR. Pregnancy in the first 15 weeks with PR -> OB referral (Drago PMID 18489054 signal). Persistent (>12 wk) atypical eruption -> biopsy + RPR + drug review. New onset in an immunosuppressed patient -> biopsy to exclude atypical viral / lymphomatoid mimics.
   inputs: palm_or_sole_involvement_or_mucosal_lesions
   actions: panel.cbc
   advance: syphilis red flags + pregnancy / persistence / immunosuppression screened and active workup ordered if present
5. 5INITIAL_WORKUP
   CLASSIC TYPICAL PR (herald patch + Christmas-tree + trunk-only + no palm/sole + no mucosa + low STI risk): NO laboratory tests required — diagnosis is clinical. ATYPICAL / PALM/SOLE / SEXUALLY-ACTIVE / PERSISTENT / PREGNANT: order RPR (or VDRL) + treponemal test (FTA-ABS / TPPA / EIA / CIA), KOH scrape if tinea suspected, +/- skin biopsy if persistent (>12 wk), atypical morphology, drug-induced suspicion, or mycosis-fungoides concern (poikilodermatous / fixed lesions). HHV-6/7 PCR is research-grade and NOT routinely indicated outside pregnancy (Drago JAAD 2008 PMID 18489054).
   inputs: rpr_and_treponemal_serology, koh_microscopy_of_scale
   actions: panel.cbc
   advance: classic PR -> no tests; atypical -> RPR + treponemal + KOH + biopsy as indicated
6. 6BRANCHING_WORKUP
   Diagnosis branch: classic + low STI risk + no palms/soles -> PR confirmed clinically. RPR-reactive -> SECONDARY SYPHILIS pathway (NOT PR — treat per CDC STI guidelines: benzathine penicillin G 2.4 million units IM x1; partner notification; HIV co-testing; route OUT). KOH+ -> tinea corporis (route topical / oral antifungal). Recent drug temporal link + atypical histology (necrotic keratinocytes, eosinophils, interface dermatitis) -> pityriasiform drug eruption (route derm.drug-eruption.core.v1). Fixed / poikilodermatous / persistent patches -> biopsy for mycosis fungoides / pityriasis lichenoides chronica.
   inputs: duration_and_recurrence_status
   actions: workup.chronic_pruritus
   advance: specific diagnosis assigned; alternatives routed by engine_id; PR confirmed if all branches negative
7. 7DIFFERENTIAL
   Terminal differential with named pivots: PR vs SECONDARY SYPHILIS (palm/sole involvement + mucous patches + lymphadenopathy + RPR-reactive pivot — the not-to-miss; mandatory rule-out) vs nummular eczema (well-defined coin-shaped plaques without collarette pivot; itch-dominant; chronic course) vs tinea corporis (annular advancing scale with central clearing, KOH-positive pivot) vs guttate psoriasis (post-streptococcal; smaller papules with thicker silvery scale; no herald patch; scalp + nail pivot — route derm.psoriasis.core.v1) vs pityriasis lichenoides chronica (chronic course with varied lesion ages; biopsy pivot) vs PITYRIASIFORM DRUG ERUPTION (temporal drug link, eosinophils + necrotic keratinocytes on biopsy — route derm.drug-eruption.core.v1) vs atypical viral exanthem (Gianotti-Crosti, viral exanthem of childhood) vs mycosis fungoides (chronic / fixed / poikilodermatous — biopsy mandatory). Always biopsy a persistent / atypical / treatment-resistant case before assuming PR.
   advance: best diagnosis selected; secondary syphilis actively excluded if any risk; drug-eruption ddx considered and routed
8. 8RISK_STRATIFICATION
   Severity = BSA × intensity × itch × QoL/sleep impact (no validated PR-specific scale). MILD = limited BSA, mild itch, no sleep disruption -> reassurance + emollient. MODERATE = pruritus disrupting sleep or daily activities -> topical low-mid corticosteroid + oral H1 antihistamine. SEVERE = extensive BSA, intense pruritus despite topicals -> consider nbUVB phototherapy or short oral corticosteroid course; some experts add aciclovir for severe/extensive disease (Ciccarese PMID 39597810 network meta-analysis ranks aciclovir highest for rash improvement; oral steroids + antihistamine highest for itch resolution). Pregnancy + first-trimester onset = high-priority OB consultation regardless of skin severity.
   inputs: pruritus_severity_and_sleep_impact
   advance: severity tier + management intensity assigned
9. 9TREATMENT
   REASSURANCE FIRST + patient education that PR is self-limited and resolves in 6-8 weeks; recurrence is uncommon (~3%); post-inflammatory pigmentary change is expected in darker skin and is also self-limited. SYMPTOMATIC MANAGEMENT: emollient + gentle skin care (non-pharm); topical low-mid potency corticosteroid (eg triamcinolone 0.1% cream BID short course for inflamed itchy plaques; hydrocortisone 2.5% for face/folds if involved); oral H1 antihistamine (cetirizine 10 mg PO daily; hydroxyzine 25 mg PO at night if sedation/sleep needed); nbUVB phototherapy for severe extensive symptomatic disease. ANTIVIRAL OPTIONAL: aciclovir / valaciclovir per Ciccarese PMID 39597810 may speed rash resolution in severe / symptomatic cases but is NOT first-line and is not antibiotic. NO ANTIBIOTICS for classic PR. PREGNANCY: confirm not drug-induced; OB referral; avoid systemic agents unless clearly indicated; topical low-potency steroid + emollient + reassurance.
   inputs: pruritus_severity_and_sleep_impact, pregnancy_status_and_gestational_age
   advance: reassurance documented; symptomatic measures + (only if needed) limited topical / oral treatment started; pregnancy gating applied
10. 10DISPOSITION
    Entirely outpatient. NO admission indicated for PR per se. Referrals: OB for pregnant patient with first-trimester onset (Drago PMID 18489054); STI clinic / primary care for confirmed secondary syphilis (NOT this engine); derm referral for atypical / persistent / treatment-resistant disease + biopsy.
    inputs: pregnancy_status_and_gestational_age
    advance: disposition documented; OB / STI / derm referrals made as indicated
11. 11MONITORING
    Review at 4-6 weeks to confirm expected resolution and absence of complications (post-inflammatory pigment change is expected and benign in darker skin). For pregnant patients: OB co-management throughout pregnancy with PR. For patients started on topical steroid: limit course (~1-2 weeks at a time on body; shorter on face/folds) to avoid atrophy. For patients on oral antihistamine: monitor sedation effects and avoid driving with hydroxyzine.
    advance: expected resolution confirmed by 8-12 weeks; persistence -> back to BRANCHING_WORKUP
12. 12FOLLOWUP
    Confirm resolution by 8-12 weeks. PERSISTENT (>12 weeks) or RECURRENT eruption -> re-evaluate: repeat RPR, drug review, biopsy for pityriasis lichenoides chronica / drug-induced eruption / mycosis fungoides. Counsel that recurrence is uncommon (~3%) but can happen; post-inflammatory pigmentary changes may persist for months and are not a sign of treatment failure; pregnant patients with first-trimester PR need OB continuity through delivery.
    inputs: duration_and_recurrence_status
    actions: workup.chronic_pruritus
    advance: resolution confirmed; persistence triggers re-workup; pregnancy continuity arranged