**Question:** Milk is a poor source of which of the following?

**Core Concept:** Milk is a nutritious food derived from mammals, containing essential nutrients like proteins, carbohydrates, fats, vitamins, and minerals. However, certain nutrients are present in lower amounts or not efficiently absorbed by the human body.

**Why the Correct Answer is Right:**
- Option A is incorrect because calcium is a major component of milk, and it is an essential nutrient for bone health.
- Option B is also incorrect as milk contains vitamin D, which helps in the absorption of calcium; however, the body can't efficiently produce vitamin D from sunlight alone and relies on dietary intake, making milk a significant source.
- Option C is incorrect because iron is present in milk, but its absorption is hindered by the presence of phytic acid and other components like calcium and casein.
- Option D is incorrect as protein content in milk is not poor, but the amino acid profile is not optimal for human beings.

**Why Each Wrong Option is Incorrect:**
- Option A (calcium): Although calcium is present in milk, its absorption is hindered by the presence of phytic acid and other components like calcium and casein.
- Option B (vitamin D): While vitamin D is present in milk, the body can't efficiently produce vitamin D from sunlight alone, making milk a significant source.
- Option C (iron): Iron in milk is also hindered by the presence of phytic acid and other components like calcium and casein.
- Option D (protein): The protein content in milk is not poor, but the amino acid profile is not optimal for human beings.

**Clinical Pearl:** To optimize iron absorption from milk, it is recommended to consume it with foods rich in vitamin C, like citrus fruits or bell peppers, which can enhance iron absorption.

**Correct Answer:** Option C (iron)