(Fr: PINS aiguë). Acute, generally bilateral and diffuse pneumonitis. More rapid in onset and/or denser, more extensive and severe than pattern Ib. Can be fulminate in the form of diffuse pulmonary interstitial or alveolar opacities and the ARDS picture (see under IIb). BAL is generally lymphocytic and is also indicated to rule out pneumonia due to Pneumocystis, BCG, viruses or other agents particularly in the immunodepressed. Pathology (lung biopsy is not necessary in the majority; before going for the biopsy, please look at PMID 25950989) may disclose dense NSIP, widespread granulomas, or OP depending both on patient and drug. In severe cases pulmonary edema, DAD and/or DAH may develop. Corticosteroid therapy is indicated if patients progress to acute respiratory failure, once an infection has been carefully and resonably ruled out. Transition to pulmonary fibrosis is very uncommon. The boundary between patterns Ia and Ib may be difficult to delineate. See also under IIb