Let the focus of the ellipse and the Y axis be C and D respectively. Since the topic requires that a circle with a diameter of PQ always passes through point M, then we L are these two straight lines: perpendicular to the Y axis and passing through point C or D, so that two circles with a diameter of PQ (CQ and DQ at this time) intersect with the X axis, and point M must be one of these two points and must be on the X axis. You can see it from symmetry when you draw it.