Okay, let's tackle this question. The user is asking about the standard power of an intraocular lens (IOL) implanted in the posterior chamber. First, I need to recall the typical power used for IOLs. I remember that in cataract surgery, the goal is usually to achieve emmetropia, which means the person doesn't need glasses for distance vision. The standard power is often around 20 diopters (D). But wait, I should check if there's a specific range or if it varies based on factors like the patient's eye size or pre-existing conditions.

Wait, the question mentions "posterior chamber" IOLs. Posterior chamber IOLs are the most common type used nowadays, placed in the capsular bag. The power calculation depends on the axial length of the eye, corneal curvature, and other factors, but the standard starting point is usually 20 D. However, some sources might say that the standard power is adjusted based on the patient's preoperative refraction. Hmm, maybe the answer is 20 D. But I need to confirm if there's a more precise value. Let me think about the common practice. In many cases, for a normal eye, the IOL power is calculated to correct the refractive error, but if the patient is emmetropic, the power is around 20-23 D. However, the standard power isn't fixed; it's calculated using formulas like SRK/T, Holladay, etc. But the question is about the "standard" power, implying a default value before calculations. Wait, maybe the standard power refers to the typical power used when no specific calculations are done, which might be around 20 D.

Now, looking at the options (though they're not provided here), if the correct answer is 20 D, then the explanation should clarify that the standard power is approximately 20 D for a normal eye. But I need to make sure that there isn't a more precise number. Wait, some sources mention that the average IOL power implanted is around 20-23 D for emmetropia. However, the exact value can vary. The key here is that the question is about the standard power, so the answer is likely 20 D. The other options might be incorrect because they could be numbers like 10 D (too low), 30 D (too high), or other values that don't align with standard practice. The core concept here is the typical IOL power used in posterior chamber implantation for emmetropia. The clinical pearl would be that the standard power is around 20 D, adjusted based on individual patient metrics.

**Core Concept**
Intraocular lens (IOL) power is calculated to achieve emmetropia post-cataract surgery. The standard power for posterior chamber IOLs assumes a typical axial length and corneal curvature, with adjustments made via formulas like SRK/T or Holladay.

**Why the Correct Answer is Right**
The standard power for a posterior chamber IOL in an average adult with no pre-existing refractive error is approximately **20 diopters (D)**. This accounts for the optical properties of the human eye, where the cornea contributes ~40 D of refractive power, and the IOL compensates to focus

✓ Correct Answer: B. 20 dioptres