Granuloma: This is often the most common cause of a lung nodule in a young adult without a smoking history. Granulomas can be caused by infections such as histoplasmosis or tuberculosis, and they can also be idiopathic.

Hamartoma: A benign lung tumor that is common in young adults. It is usually asymptomatic and found incidentally on imaging.
Pulmonary Arteriovenous Malformation (AVM): Although less common, AVMs can present as lung nodules and are more frequently found in younger individuals.
Infectious causes (e.g., pneumonia, abscess): Certain infections can cause lung nodules, especially if they are not fully resolved.

Lung Cancer: Although less likely in a young non-smoker, it is crucial not to miss this diagnosis. Certain types of lung cancer, like adenocarcinoma, can occur in non-smokers.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lung nodules, and early diagnosis is critical for treatment.
Metastasis: If the patient has a history of cancer elsewhere in the body, metastasis to the lung should be considered, even if it's uncommon in young adults without a smoking history.

Pulmonary Sarcoidosis: An autoimmune disease that can cause lung nodules, although it more commonly presents with bilateral hilar lymphadenopathy.
Congenital anomalies (e.g., bronchogenic cysts, sequestrations): These are rare and usually present in childhood but can occasionally be diagnosed in adults.
Inflammatory or autoimmune conditions (e.g., Wegener's granulomatosis, rheumatoid nodules): These conditions can cause lung nodules but are less common and often associated with other systemic symptoms.