**Core Concept:** Orbital biometry refers to measuring various parameters of the orbit, including axial length, corneal thickness, lens thickness, vitreous chamber depth, and central corneal thickness, which are essential for determining refractive errors and intraocular lens power calculation in cataract surgery.

**Why the Correct Answer is Right:** Orbital biometry utilizes A-mode ultrasound, also known as A-scan ultrasound, which scans the eye by measuring the sound speed and time required for the sound wave to travel through the various structures of the eye. This method is based on the principle of pulse-echo technique, where a high-pitched sound wave is sent into the eye, and the echo is received back as the wave reflects off the interfaces between the layers of the eye (e.g., the cornea, aqueous humor, lens, vitreous humor, and the retina). By analyzing the time delay and intensity of the echo, the thickness of each layer can be calculated.

**Why Each Wrong Option is Incorrect:**

A. **B-mode ultrasound (B-scan)** measures the reflection or echo intensity but does not calculate the exact thickness of the eye layers. This mode is useful in visualizing the retina, optic nerve, and the vitreous chamber but is less effective in measuring orbital structures.

B. **A-mode ultrasound (A-scan)** measures the time delay of the echo but does not differentiate between the various layers of the eye. This mode is used mainly for fetal biometry, not orbital biometry.

C. **Ultrasound biometry** is a combination of A-mode and B-mode ultrasound. It improves the accuracy of orbital biometry but is not the primary mode used for this purpose due to the limitations of A-mode and B-mode ultrasound individually.

D. **Pulse oximetry** measures the oxygen saturation of blood but is not applicable for orbital biometry as it is a technique used for assessing oxygen saturation in tissues, not biometry of the eye.

**Clinical Pearl:** Orbital biometry is mainly performed using A-mode ultrasound (A-scan), which is a valuable tool in preoperative planning for cataract surgery or intraocular lens power calculation. This information is crucial for the surgeon to determine the appropriate intraocular lens power for optimal vision restoration after cataract removal.