It's a lung disease that can't be seen by outward appearance.

I believe it is likely a tie between "lymphangioleiomyomatosis" and "laryngotracheobronchitis," each at 24 letters.

It is a benign tumor of the kidney, and as such the cause is not known. They are usually solitary and sporadic. They are also found with a rare hereditary disease, tuberous sclerosis, in which case they are usually more numerous tumors in the affected individual. They can be found in lymphangioleiomyomatosis, a very rare lung condition found only in women.

"LAM" can stand for several different things depending on the context. It could refer to "Lymphangioleiomyomatosis," a rare lung disease, or "Logical Address Mapping" in computer science. It's important to provide more information for a specific answer.