Unless revenue approved, usually, profits from share options have an income tax component and sometimes have a capital gains component.
As far as I know, if your options were at $4.00 (strike price), the shares were being traded publically at $5.00 (closing price) on the day you exercised your options (i.e. bought shares) and you received $5.50 when you sold the shares (selling price), you are liable for income tax on the difference between $4.00 (strike price) and $5.00 (closing price). You are liable for capital gains on the difference between $5.00 (closing price) and $5.50 (selling price). As I understand it, you cannot offset any capital losses or CGT allowances against the income tax component, only against the CGT component (if any) of the transaction. There's a further sting: the income tax is due 30 days after buying the options.