The soloution from user bryjamus managed to pass. So test cases like "abca" are missing. Where first and last letter are the same but is still not a palindrome.

public static bool IsPalindrome(string w)
{
var arr = w.ToLower().ToArray().Where(x=>Char.IsLetter(x));
return arr.First() == arr.Last();
}

That will return wrong answer if the first and letter charater are the same, and the letters after that is not a palindrome.
Like "abca" return true where it should return false.