A patch is available to correct this problem:
During the mushroom hunt, the inventory shows only 4 mushrooms, even though 5 have been collected.
***This update will work for both the CD and Digital Download versions of the game.***
Sometimes a mushroom has been overlooked or not actually taken and added to the inventory during the hunt. Be sure to check each location again to make sure that each mushroom has been taken and is no longer available.
If no mushrooms remain to be collected and the inventory still shows only 4 mushrooms, download and install the patch to the Legend of the Crystal Skull game folder.
If you continue to encounter any problems with the inventory not updating to show all 5 mushrooms, email email@example.com for further assistance.