**Core Concept:** Gallstones are hard, spherical formations that can develop in the gallbladder or bile ducts. They are composed of bile pigments, cholesterol, and bilirubin salts.

**Why the Correct Answer is Right:** Gallstones are primarily composed of cholesterol and bilirubin salts, which are precipitated from the bile due to alterations in bile composition or supersaturation. In most cases, the presence of bile pigments like bilirubin and cholesterol leads to the formation of gallstones.

**Why Each Wrong Option is Incorrect:**

A. Gallstones can contain cholesterol, making this option incorrect because it contradicts the primary composition of gallstones.
B. Gallstones may contain pigments like bilirubin, but they do not exclusively consist of pigments. This option is incorrect because the primary components are cholesterol and bilirubin salts, not pigments.
C. Gallstones can contain calcium salts, but they primarily consist of cholesterol and bilirubin salts. This option is incorrect because calcium salts are not the main component of gallstones.
D. Gallstones can contain water and electrolytes, which are components of bile but do not define the primary composition of gallstones. This option is incorrect as these substances are present in bile but not the primary components of gallstones.

**Clinical Pearl:** Understanding the composition of gallstones is crucial for clinical practice, as they can cause complications like cholecystitis, cholangitis, and cholecystectomy.

**Correct Answer:** D. Gallstones do not contain water and electrolytes, which are components of bile but not the primary components of gallstones.