**Core Concept**
Amsler's sign is a clinical indicator of a specific condition affecting the eye. It is related to the presence of a particular type of cataract that affects the lens of the eye.

**Why the Correct Answer is Right**
Amsler's sign is seen in posterior polar cataract, which is a rare congenital or developmental cataract. The sign is characterized by a small, well-defined, round or oval opacity in the posterior part of the lens. This opacity is usually unilateral and may be asymptomatic until later in life. The exact mechanism behind Amsler's sign is not well understood, but it is thought to be related to the abnormal development of the lens fibers during embryogenesis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Amsler's sign is not typically associated with cortical cataracts, which are characterized by spoke-like opacities in the lens.
* **Option B:** This option is incorrect because Amsler's sign is not seen in congenital glaucoma, which is a condition characterized by increased intraocular pressure in infants and young children.
* **Option C:** This option is incorrect because Amsler's sign is not typically associated with retinal detachment, which is a condition where the retina separates from the underlying tissue.

**Clinical Pearl / High-Yield Fact**
Amsler's sign is an important clinical indicator that should be recognized by ophthalmologists and other healthcare professionals. It is essential to remember that Amsler's sign is a specific sign associated with posterior polar cataract, and it should not be confused with other conditions that may present with similar symptoms.

**Correct Answer: D. Posterior polar cataract**