一名二十多岁、有吸烟史的男性患者主诉反复出现气急症状。近4个月来，他因类似症状已经3次住院，尽管接受了多次抗生素治疗。在这次就诊中，患者体温正常，右肺闻及呼吸音减弱。胸部CT显示右中叶和下叶有大量实变，与4个月前相比病情恶化。感染性检查包括支气管镜检查和支气管肺泡灌洗未能确定致病微生物。免疫性疾病检查结果阴性。经纤支镜活检最终确定为分化良好的3b期肺腺癌，KRAS G12C突变阳性。患者已被转诊至肿瘤科门诊进行随访，目前已开始化疗。该病例突出了年轻患者诊断偏见的问题，并强调了在临床预期改善不明显的疑似社区获得性肺炎中，支气管镜活检对确诊的重要性。© BMJ Publishing Group Limited 2023. 未经商业再利用。查看权限。BMJ出版。

A man in his 20s with a history of tobacco use presented with recurrent shortness of breath. He was hospitalised three times within the past 4 months for similar symptoms despite completing several courses of antibiotic therapy. In this presentation, he was afebrile with rhonchi and decreased breath sounds over the right lung. Chest CT demonstrated large consolidations in the right middle and lower lobes, worsened compared with 4 months prior. Infectious workup including bronchoscopy with bronchoalveolar lavage did not identify a causative organism. Testing for immune disease was negative. Transbronchial biopsy ultimately identified well-differentiated stage 3b lung adenocarcinoma with a KRAS G12C mutation. The patient was referred to oncology for outpatient follow-up and has since initiated chemotherapy. This case highlights diagnostic biases encountered in young patients and the utility of bronchoscopic biopsy for definitive diagnosis in presumed community-acquired pneumonia when the clinical outcome is not improving as expected.© BMJ Publishing Group Limited 2023. No commercial re-use. See rights and permissions. Published by BMJ.