**Core Concept**
Visual acuity testing in infants is challenging due to their inability to communicate or respond to traditional visual acuity tests. Landolt's rings are a modified version of the familiar Snellen chart, designed to assess visual acuity in young children and infants by detecting their ability to perceive a gap or a break in the ring.

**Why the Correct Answer is Right**
Landolt's rings consist of a series of rings with a gap or break in them, which are presented to the infant at various distances. The infant's ability to detect the gap or break is used as an indicator of their visual acuity. This method is particularly useful in infants because it does not require them to read or recognize letters, making it a more objective and reliable measure of their visual acuity. The use of Landolt's rings in visual acuity testing in infants is based on the principle that even young children can detect a break in a ring, which is a fundamental aspect of visual perception.

**Why Each Wrong Option is Incorrect**
**Option B:** The '4' dot test is a type of visual acuity test used in adults, where the subject is asked to identify the orientation of four dots. This test is not suitable for infants, who are unable to communicate their responses.
**Option C:** A perimeter is a device used to measure the visual field, which is the area that an individual can see when looking straight ahead. While perimeters are essential in ophthalmology, they are not used to test visual acuity in infants.
**Option D:** A slit lamp is a bio-microscope used to examine the anterior segment of the eye, including the cornea, iris, and lens. While slit lamps are crucial in ophthalmology, they are not used to test visual acuity in infants.

**Clinical Pearl / High-Yield Fact**
Infants as young as 6 months old can detect a break in a Landolt's ring, indicating that their visual acuity is better than previously thought. This is an essential fact for pediatric ophthalmologists to keep in mind when assessing visual acuity in young children.

**✓ Correct Answer: A. Landolt's rings**