Benign granuloma: This is the most common cause of small subpleural nodules, often resulting from healed infections such as histoplasmosis or tuberculosis. The small size of the nodule (3.8 mm) and its subpleural location are consistent with a benign granuloma.

Hamartoma: A benign lung tumor that can appear as a small nodule, especially in the subpleural region. While less common than granulomas, hamartomas are still a relatively frequent finding.
Intrapulmonary lymph node: These can be found in the subpleural region and may appear as small, soft tissue density nodules. They are more common in certain conditions but can be incidental findings.
Small metastasis: Although less likely given the small size, a solitary metastasis from a primary malignancy elsewhere in the body could present as a subpleural nodule.

Lung cancer: Even though the nodule is small, it is crucial not to miss early lung cancer, which can present as a solitary subpleural nodule. The prognosis significantly improves with early detection.
Infectious nodules (e.g., tuberculosis, fungal infections): Certain infections can cause nodules that may appear similar to benign lesions but require prompt treatment to prevent progression and complications.
Vascular malformation or anomalous vessel: Although rare, these can mimic soft tissue nodules and are important to identify due to potential risks of bleeding or other complications.

Amyloidosis: Can cause nodular deposits in the lung, including in a subpleural location, though this is less common.
Lymphangitic carcinomatosis: This condition, characterized by the spread of cancer through lymphatic vessels in the lung, can sometimes present with nodular lesions, though it typically involves more extensive disease.
Sarcoidosis: While sarcoidosis often presents with bilateral hilar lymphadenopathy, it can also cause pulmonary nodules, including in the subpleural region, though this is less typical for the disease.