Remember when everyone thought Senator John McCain was getting senile during the 2008 presidential race? Statements such as "John McCain isn't the sharpest knife in the drawer" resulted in millions of nervous Americans casting their votes for his opponent, Senator Barack Obama.

But shortly after Obama's historic landslide election in 2008, clues began to emerge that he wasn't the brightest bulb in the pack either.

For instance, while giving a speech in London, England in 2009, Mr. Obama forgot where he met his wife, Michelle Obama.

Coincidentally, the most recent sign of a disturbing lapse in Mr. Obama's memory occurred in London again. This time, Mr. Obama forgot what year it was when he signed a guestbook in the nave of Westminster Abbey yesterday.

According to London's Daily Mail: instead of signing '2011' after his name in the guestbook, Obama wrote '24 May 2008' (it is customary for the British to write the day first followed by the month and year). He got the date wrong by three years when he signed the guestbook - despite apparently asking the dean what day it was.

"DOESN'T OBAMA KNOW WHAT YEAR IT IS?" shouts the embarrassing headlines in the London papers today.

In the first stage of Alzheimer's, a disease that destroys brain cells, there is no visible impairment. In stage 2, the individual begins to experience memory lapses, forgetting familiar words or locations. In stage 3, there is a noticeable cognitive decline and the person has trouble remembering names and dates. The symptoms develop slowly and get worse over time.

It is believed that former president Ronald Reagan suffered from Alzheimer's while he was still in office.

Click this link to see more beautiful pictures of the Obamas' official UK trip!