**Question:** Iron absorption is increased by

A. Vitamin C
B. Proton pump inhibitors
C. Cholestyramine
D. Calcium

**Correct Answer:** A. Vitamin C

**Core Concept:**
Iron is an essential trace element required for the production of hemoglobin, myoglobin, and certain enzymes in the human body. Iron absorption occurs primarily in the duodenum, particularly in the duodenal bulb and the second part of the duodenum. There are two main pathways for iron absorption: the divalent cation transporter (DMT1) and the ferroportin-hephaestin system. Vitamin C (ascorbic acid) enhances iron absorption by increasing the solubility of iron and promoting its incorporation into the DMT1 transporter.

**Why the Correct Answer is Right:**
Vitamin C, also known as ascorbic acid, is a water-soluble antioxidant that plays a critical role in collagen synthesis and scavenging free radicals. Its role in iron absorption is due to its ability to increase the solubility of iron ions (Fe²⁺) in the gastrointestinal tract. When absorbed, these soluble iron ions can be incorporated into the divalent cation transporter (DMT1) and the ferroportin-hephaestin system for transport across the enterocyte membrane and into the bloodstream.

**Why Each Wrong Option is Incorrect:**

B. Proton pump inhibitors (e.g., omeprazole, pantoprazole) are medications that reduce gastric acid secretion, inhibiting the gastric proton pump (H⁺/K⁺ ATPase) located in the parietal cells of the stomach. Proton pump inhibitors do not directly affect iron absorption.

C. Cholestyramine is a resin that binds bile acids, sequestering them in the gut, which ultimately reduces reabsorption of cholesterol and fat-soluble vitamins (e.g., A, D, E, and K). Cholestyramine does not directly impact iron absorption.

D. Calcium is an essential mineral involved in bone health, neuromuscular function, and blood clotting. Calcium does not have a direct impact on iron absorption.

**Clinical Pearl:**
Iron supplementation is often prescribed for patients with iron deficiency anemia. When administering iron supplements, it is essential to consider the potential interactions with other medications, such as proton pump inhibitors and cholestyramine. In such cases, these medications can interfere with the absorption of fat-soluble vitamins and bile acids, which may lead to reduced dietary iron absorption. On the other hand, ensuring adequate vitamin C intake is crucial for optimal iron absorption, as it helps to solubilize dietary iron, making it more accessible for absorption by the enterocytes.