Final answer:
The legal environment is the factor outside the control of the external auditor that affects audit quality.
Step-by-step explanation:
The factor outside the control of the external auditor that affects audit quality would be the legal environment. While organizational corporate governance, systems for making client acceptance decisions, and the regulatory environment are all important factors that can impact audit quality, they are within the control or influence of the external auditor to some extent. However, the legal environment, including laws and regulations related to auditing, is determined by external entities, such as government bodies, and the auditor has no control over it.