**Core Concept**
The presence of bilateral lung infiltrates on a chest X-ray suggests a respiratory or infectious process, warranting further investigation to determine the underlying cause.

**Why the Correct Answer is Right**
To determine the cause of bilateral lung infiltrates, a high-resolution computed tomography (HRCT) scan of the chest is often the next step. HRCT provides more detailed images of the lung parenchyma, allowing for better assessment of the extent and distribution of lung infiltrates. This can help differentiate between various causes such as pneumonia, interstitial lung disease, or pulmonary edema.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood gas analysis may provide information on the oxygenation status and acid-base balance but does not directly image the lung infiltrates. It is a useful adjunct but not the next investigation.
**Option B:** Ultrasound is useful for evaluating pleural effusions or pneumothorax but not for assessing lung parenchymal disease.
**Option C:** Pulmonary function tests (PFTs) can provide information on lung function but do not directly image the lung infiltrates.

**Clinical Pearl / High-Yield Fact**
Remember that HRCT is the imaging modality of choice for evaluating lung infiltrates, especially when the diagnosis is unclear or when interstitial lung disease is suspected.

**Correct Answer: C. Pulmonary function tests (PFTs) can provide information on lung function but do not directly image the lung infiltrates.