The "a" of the penta- prefix is not dropped before a vowel. As the IUPAC Red Book 2005 page 69 states, "The final vowels of multiplicative prefixes should not be elided (although monoxide, rather than monooxide, is an allowed exception because of general usage)."
There are a number of exceptions and special cases that violate the above rules. Sometimes the prefix is left off the initial atom: IO is known as iodine pentaoxide,  but it should be called diiodine pentaoxide. NO is called nitrogen sesquioxide (sesqui- means ).
The main oxide of phosphorus is called phosphorus pentaoxide. It should actually be diphosphorus pentaoxide, but it is assumed that there are two phosphorus atoms (PO), as they are needed in order to balance the oxidation numbers of the five oxygen atoms. However, people have known for years that the real form of the molecule is PO, not PO, yet it is not normally called tetraphosphorus decaoxide.
In writing formulas, ammonia is NH even though nitrogen is more electronegative (in line with the convention used by IUPAC as detailed in Table VI of the red book). Likewise, methane is written as CH even though carbon is more electronegative (Hill system).