From: Rabbi Moshe Sofer
To: Rabbi Avraham David Rubinstein
The Chasam Sofer rules stringently in line with the Acharonim that unexplained whitish discoloration on a lung renders an animal treif, but carves out a practical leniency: if the surface membrane shows only a faint shimmer while the interior is more intensely white, one should peel off the pleural membrane and re-examine; if the whiteness then disappears, the discoloration was merely an internal condition showing through, and since an internal deficiency is not a halachic deficiency, the lung may be deemed kosher.
Source: Shu"t Chasam Sofer YD §34