**Core Concept**
Ectopia lentis refers to the displacement or malposition of the lens in the eye, which can cause various visual disturbances, including uniocular diplopia. This condition often results from trauma, congenital anomalies, or systemic diseases that affect the supportive structures of the lens, such as the zonular fibers.

**Why the Correct Answer is Right**
The golden crescent sign and dark crescent line observed with oblique and coaxial illumination, respectively, are characteristic findings in ectopia lentis. The golden crescent sign occurs when light passes through the displaced lens and is partially absorbed by the iris, creating a golden hue. The dark crescent line is visible when light is blocked by the iris, creating a shadow. This distinctive appearance is due to the malpositioned lens, which disrupts the normal anatomy of the anterior segment.

**Why Each Wrong Option is Incorrect**
**Option A:** Lenticonus refers to a conical protrusion of the lens, which can cause visual disturbances but does not typically result in the characteristic signs of ectopia lentis.

**Option B:** Coloboma is a congenital defect of the eye, including the iris, retina, or choroid, and is not directly associated with the displacement of the lens.

**Option C:** Microspherophakia is a condition characterized by a small, spherical lens, which can cause visual disturbances but is not typically associated with the characteristic signs of ectopia lentis.

**Clinical Pearl / High-Yield Fact**
The "oblique illumination" test is a valuable clinical tool for diagnosing ectopia lentis, as it can help distinguish this condition from other causes of uniocular diplopia. Recall that a golden crescent sign in conjunction with a dark crescent line is highly suggestive of ectopia lentis.

**Correct Answer:**
✓ Correct Answer: D. Ectopia lentis.