When measuring a patient with a horizontal deviation, the Maddox rod should be positioned so that the line forms a _________.

The Maddox rod is a diagnostic tool used in the evaluation of strabismus, which is a condition characterized by misalignment of the eyes. When addressing a patient with a horizontal deviation, the correct positioning of the Maddox rod is critical for accurate measurement.

Positioning the rod to create a vertical line allows the examiner to assess the relative alignment of the patient's eyes when viewing a light source. In situations where there is a horizontal deviation, the vertical line produced by the Maddox rod indicates how the eyes respond to the light target, helping to determine the nature and extent of the deviation. This vertical alignment allows the clinician to measure the angle of deviation more effectively, providing insights into the patient's ocular alignment.

Other options, such as a horizontal line, diagonal line, or circular line, do not conform to the established protocol for measuring horizontal deviations, and such configurations would not yield reliable or interpretable results in this context.