Sin, Sun, stain, skin, spin, spun, span, scan, shun...
sanitation
Lighthearted is a 12 letter word. Lithographed is another 12 letter word that starts with L and ends with D.
Arithmetic
The longest English word that starts with 'l' and ends with 'd' is "linguistically." It has 12 letters and pertains to language or linguistics. However, if you're specifically looking for a word that fits the criteria and is a common usage term, "languished" is another option, though it's shorter at 10 letters.
The only English word I could find that meets the criteria is candleholder, but I am not sure it is a hospital helper.
Suffragettes.
handsomenessharmlessnesshelplessnesshomelessnesshopelessnesshorribleness
denomination
lsjfl
* contribution * colonization * cancellation * civilization * constitution * commendation * compensation * condensation * congregation * confirmation * conversation * conservation * constipation * continuation * coordination
exaggerative, exhilarative exaggerative, exhilarative
Exterminable
chauvinisticcatastrophic