Cushing's syndrome due to adrenocorticotropic hormone-secreting metastatic neuroendocrine tumor of unknown primary origin: a case report and literature review.

This is a challenging case of UPO NEN presenting with ECS and confounding factors, such as previous infection and incidental lesions, during the diagnosis process. The case in question highlighted the fact that atypical pulmonary carcinoid with a low proliferation index may cause visible metastases even when radiologically undetectable.