Okay, let's tackle this question about the radioopacity of gallstones. The user wants a detailed explanation for a medical student preparing for exams like NEET PG or USMLE. First, I need to recall the core concept here. Gallstones can be cholesterol, pigment, or mixed. Their radioopacity depends on their composition.

The correct answer is probably around 10-15%, since cholesterol stones are radiolucent and make up most cases. Pigment stones, which are more radioopaque, are less common. So the core concept is the composition of gallstones affecting their visibility on X-rays.

Now, why is the correct answer right? Cholesterol stones are mainly composed of cholesterol, which doesn't show up on X-rays. Only about 10-15% of gallstones contain enough calcium to be visible. Pigment stones have bilirubin and calcium, making them radioopaque. But since most gallstones are cholesterol, the percentage is low.

For the wrong options, if the options include higher percentages like 50% or 80%, those are incorrect because they overestimate the radioopacity. If there's a lower percentage like 5%, that's too low. Another wrong option might be 100%, which is obviously wrong since not all are visible. Each distractor needs a brief explanation.

The clinical pearl here is that most gallstones are radiolucent, so X-ray isn't the best imaging modality. Ultrasound is preferred. Also, the presence of radioopaque stones is a classic sign in some conditions but isn't the main diagnostic tool.

I need to structure this into the required sections, making sure each part is concise and fits within the character limit. Avoid jargon but use precise terms. Check that each wrong option is addressed clearly, and the correct answer is explained with the right mechanisms. Finally, end with the correct answer line.

**Core Concept**
Radioopacity of gallstones depends on their composition. Cholesterol stones are radiolucent, while pigment stones (containing calcium bilirubinate) are often radioopaque. Only a minority of gallstones contain sufficient calcium for X-ray visibility.

**Why the Correct Answer is Right**
Approximately **10–15% of gallstones** are radioopaque. These are typically **pigment stones** (brown or black), which contain calcium salts of bilirubin. Cholesterol stones, the most common type (80% of cases), are radiolucent due to their cholesterol-rich, calcium-poor composition. Radioopacity occurs when stones have ≥30–40% calcium content, enabling X-ray detection. This explains the low overall percentage of radioopaque gallstones.

**Why Each Wrong Option is Incorrect**
**Option A:** 50% is incorrect—this overestimates the proportion of radioopaque stones, which are rare due to the dominance of cholesterol stones.
**Option B:** 80% is incorrect—reflects the prevalence of *all* gallstones but ignores their radiolucency.
**Option D:** 100% is incorrect—clearly wrong since most gallstones are cholesterol-based and X-ray invisible.

**Clinical Pearl / High-Yield Fact**
**"Most gallstones are radiolucent!"** Ultrasound is the gold standard for detection, not plain X-rays. A "positive" (radioopaque)

✓ Correct Answer: A. 20-Oct