Pityriasis Lichenoides By Shinjita Das, MD, Harvard Medical School Reviewed / Revised Aug 2021 | Modified Sep 2022 View Patient Education Diagnosis Treatment Topic Resources Acute Pityriasis Lichenoides Pityriasis Lichenoides Pityriasis lichenoides is a clonal T-cell disorder that may develop in response to foreign antigens (eg, infections or drugs) and may be associated with cutaneous T-cell lymphoma.