The most likely diagnosis is acute hypersensitivity pneumonitis (HP). HP is the result of an immunologic response to repetitive inhalation of antigens. Acute HP presents within 48 hours of a high-level exposure and is often associated with fever, flulike symptoms, cough, and shortness of breath. Radiographic imaging can demonstrate bilateral hazy opacities, and high-resolution CT imaging of the chest shows findings of ground-glass opacities and centrilobular micronodules that are upper- and mid-lobe predominant. Symptoms typically wane within 24 to 48 hours after removal from the exposure. Recurrence of symptoms with exposure to the respiratory antigen is the hallmark of this disorder, and careful attention to the history helps identify the cause. This patient presents with a known or likely exposure to a possible antigen related to his work as a timber trimmer (exposure to agricultural dusts that can readily become colonized with mold), symptoms occurring shortly after possible exposure, a history of similar symptoms, and inspiratory crackles on examination. This constellation of signs and symptoms in conjunction with the radiographic appearance makes HP the most likely diagnosis.

Acute interstitial pneumonia (AIP) is an uncommon form of diffuse lung injury that develops in response to an often unknown insult over a short period of time (days to weeks). Clinically, AIP presents with findings similar to the acute respiratory distress syndrome (ARDS), with severe and progressive hypoxia and imaging showing diffuse bilateral air space opacification. Although the time course of this patient's presentation might be consistent with this diagnosis, his other clinical findings do not support a diagnosis of AIP.

Idiopathic pulmonary fibrosis typically presents in older individuals, and its presentation is more chronic, usually with symptom duration greater than 6 months at the time of presentation. It is therefore not a likely diagnosis in this patient.

Although nonspecific interstitial pneumonia presents in younger patients such as this one, it typically presents as a chronic, rather than acute, process.