**Core Concept**
Cholesterol gallstones, also known as cholelithiasis, are formed when there is an imbalance between the secretion of bile salts, cholesterol, and phospholipids in the bile. Lithogenic bile, or bile prone to stone formation, is characterized by an increased cholesterol concentration and a decreased phospholipid concentration.

**Why the Correct Answer is Right**
A low-calorie and cholesterol-rich diet can lead to lithogenic bile, as it increases the cholesterol concentration in the bile. This is because cholesterol is more concentrated in the bile compared to the phospholipids, leading to a higher cholesterol-to-phospholipid ratio. This imbalance can facilitate the formation of cholesterol gallstones. The liver secretes more cholesterol into the bile to compensate for the low-calorie diet, while the phospholipid secretion remains relatively constant.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased biliary secretion of cholesterol would lead to an increase in phospholipid secretion to maintain the cholesterol-to-phospholipid ratio, making it a protective mechanism against lithogenic bile.

**Option B:** HMG CoA reductase is an enzyme involved in the synthesis of cholesterol in the liver. Inhibiting this enzyme with statins, not increasing its activity, is a common treatment strategy for reducing cholesterol levels in the blood and preventing lithogenic bile.

**Option C:** Clofibrate is a fibric acid derivative used to lower triglyceride levels in the blood. While it can increase bile acid synthesis, it is not a direct mechanism for forming lithogenic bile.

**Clinical Pearl / High-Yield Fact**
A low-calorie and cholesterol-rich diet can increase the risk of cholesterol gallstones by altering the cholesterol-to-phospholipid ratio in the bile. Patients with a history of gallstones should be advised to follow a balanced diet that is neither too low in calories nor too high in cholesterol.

**✓ Correct Answer: D. Low calorie and cholesterol rich diet**