Could this not be moved out into an application helper rather than in each presenter? As the page is cached so not sure of the value of using the instance variable.

Something like:

  def markdown(string)
      markdown = Redcarpet::Markdown.new(Redcarpet::Render::XHTML,
                                                   autolink: true,
                                                   space_after_headers: true,
                                                   tables: true,
                                                   superscript: true)
      markdown.render(string).html_safe
    end