def METHOD_NAME(response: Dict[str, Any], api_key: str) -> Dict[str, Any]:
    """
    If we're using a development Lob API key (one that starts with 'test_'),
    letters aren't actually mailed, but their tracking numbers returned by
    Lob still look real (at least to most people), which could confuse
    people using our demo site and such.  So we'll munge it so it definitely
    looks fake, e.g.:
        >>> _munge_tracking_number_when_fake({'tracking_number': '123'}, 'test_abc')
        {'tracking_number': 'FAKE_123'}
    However, if the API key is real, we won't munge it:
        >>> _munge_tracking_number_when_fake({'tracking_number': '123'}, 'live_abc')
        {'tracking_number': '123'}
    """
    if response.get("tracking_number") and api_key.startswith("test_"):
        response["tracking_number"] = "FAKE_" + response["tracking_number"]
    return response