**Core Concept**
Iron is an essential micronutrient for various physiological processes, including oxygen transport and DNA synthesis. In medical practice, it's crucial to identify reliable sources of iron, especially in patients with iron deficiency anemia. The "poor man's iron" refers to a readily available and inexpensive source of iron.

**Why the Correct Answer is Right**
The correct answer is **Milk of Magnesia**. Milk of Magnesia, an over-the-counter antacid, contains magnesium hydroxide, which can provide a significant amount of elemental iron. When ingested, magnesium hydroxide reacts with stomach acid to release iron, making it a useful source of iron in emergency situations or when other sources are unavailable. This is because magnesium hydroxide has a pH-dependent release of iron, which is beneficial in scenarios where rapid iron supplementation is necessary.

**Why Each Wrong Option is Incorrect**
**Option A:** Cast Iron - While cast iron cookware can leach iron into food, it's not a direct source of iron for immediate consumption.
**Option B:** Black Pepper - Black pepper contains a small amount of iron, but it's not a reliable or significant source.
**Option C:** Baking Soda - Baking soda is alkaline and can help neutralize stomach acid but does not provide a significant amount of iron.

**Clinical Pearl / High-Yield Fact**
In emergency situations, Milk of Magnesia can be used as a makeshift iron supplement, but it's essential to note that it may cause gastrointestinal side effects and should be used under medical supervision.

**Correct Answer: D. Milk of Magnesia**