What test primarily distinguishes a phoria from a tropia?

• A. Alternating cover test
• B. Unilateral cover test
• C. Maddox rod testing
• D. NPC

Answer: (B)

The main idea is that a tropia is a visible, manifest misalignment, while a phoria is latent and only shows up when fusion is broken. The unilateral cover test is the test that best reveals this distinction by isolating one eye and checking for a realignment response of the other eye when the cover is placed and removed. If a constant deviation exists (a tropia), you'll see the eyes move to re-fixate or drift when the cover is applied or removed, indicating a misalignment that is present even with one eye occluded. If no such movement occurs during this test, the deviation is latent and would only appear when fusion is disrupted in a different way, such as with an alternating cover test.

In contrast, the alternating cover test disrupts fusion repeatedly to reveal a phoria, and Maddox rod testing and the NPC test assess different aspects of binocular vision (angle of deviation under dissociation and convergence, respectively) rather than definitively distinguishing latent from manifest deviations.