A patient with breathlessness shows Kerley B-lines in chest X-ray. These lines represents:

In pulmonary edema, as the pulmonary venous pressure increases, fluid leaks from the pulmonary capillaries into the adjacent interstitium. This interstitial pulmonary edema may be identified by peribronchial cuffing, indistinctness of the perivascular margins, perihilar haziness, and thickening of the interlobular septa and interlobar fissures. Septal thickening results in Kerley A (arrow) and Kerley B (arrowhead) lines on chest radiograph. Ref: Chiles C., Gulla S.M. (2011). Chapter 4. Radiology of the Chest. In M.Y. Chen, T.L. Pope, D.J. Ott (Eds), Basic Radiology, 2e.