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The ileum, which is the final segment of the small intestine, plays a crucial role in the absorption of specific nutrients, including iron and bile salts. As food passes through the ileum, specialized cells facilitate the uptake of these essential substances into the bloodstream.

Iron absorption primarily occurs in the duodenum and proximal jejunum, but the ileum is particularly important for recycling bile salts, which are critical for fat digestion. Bile salts are reabsorbed in the ileum and then returned to the liver via the enterohepatic circulation. This process allows the body to conserve bile salts and ensures their availability for future digestion.

In contrast, the duodenum, while involved in the initial stages of digestion and nutrient absorption, does not specialize in iron and bile salts absorption the way the ileum does. The colon primarily absorbs water and electrolytes and is not involved in the absorption of iron or bile salts. The stomach focuses on the mechanical and chemical breakdown of food rather than absorption of these specific nutrients.

This understanding highlights the ileum's indispensable role in nutrient absorption within the gastrointestinal system, making it the correct answer for this question.