**Core Concept**
Pulmonary venous hypertension (PVH) is a condition where there is elevated pressure in the pulmonary veins, leading to congestion and fluid buildup in the lungs. This condition can be caused by various factors, including left-sided heart failure, mitral stenosis, or pulmonary embolism. The pathophysiology of PVH involves increased pressure in the left atrium, which subsequently backs up into the pulmonary veins.

**Why the Correct Answer is Right**
The earliest feature of PVH is peripheral pulmonary venous congestion, which is characterized by Kerley B lines on chest radiographs. Kerley B lines are short, horizontal lines at the lung periphery, representing interstitial edema. This occurs due to the increased hydrostatic pressure in the pulmonary veins, causing fluid to leak out into the interstitial spaces of the lungs. The increased pressure in the pulmonary veins also leads to enlargement of the pulmonary veins, which can be visualized on chest radiographs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Kerley B lines are not the earliest feature of PVH. They are a sign of established PVH.
**Option B:** This option is incorrect as it does not accurately describe the earliest feature of PVH. Pleural effusion is a sign of more advanced PVH.
**Option C:** This option is incorrect as it refers to a different condition. Pulmonary edema is a sign of acute respiratory distress syndrome (ARDS) or cardiogenic shock, not PVH.

**Clinical Pearl / High-Yield Fact**
Kerley B lines are a classic radiographic sign of pulmonary venous hypertension. Remember to look for short, horizontal lines at the lung periphery when evaluating a patient with suspected left-sided heart failure.

**Correct Answer: A. Kerley B lines.**