**Core Concept**
Cereal proteins, such as those found in wheat, rice, and corn, are often deficient in a specific essential amino acid that is typically abundant in animal-based protein sources. This deficiency can lead to impaired protein synthesis and overall health issues if not addressed through dietary supplementation or modification.

**Why the Correct Answer is Right**
The primary amino acid lacking in cereal proteins is **lysine**. Lysine is an essential amino acid, meaning that it cannot be synthesized by the human body and must be obtained through the diet. Cereal proteins generally have lower lysine content compared to animal-based proteins like milk, eggs, or meat. This deficiency can be particularly problematic for individuals relying heavily on cereal-based diets, such as vegetarians or vegans, who require adequate lysine intake for optimal health.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **methionine** is not the primary amino acid lacking in cereal proteins, although it can also be limited in some plant-based sources.
**Option B:** **Threonine** is an essential amino acid, but it is not the primary amino acid that cereal proteins are deficient in.
**Option C:** **Tryptophan** is another essential amino acid, but it is not the primary amino acid lacking in cereal proteins.

**Clinical Pearl / High-Yield Fact**
It's essential to note that cereal proteins can be complemented with other plant-based sources to achieve a balanced amino acid profile. For example, combining whole grains with legumes like beans or lentils can provide a more complete protein source.

**Correct Answer:** D.