**Core Concept**
Cereal proteins are a type of dietary protein derived from grains such as wheat, rice, and barley. These proteins are typically low in one essential amino acid, which is crucial for human health.

**Why the Correct Answer is Right**
Cereal proteins are deficient in the essential amino acid lysine. Lysine is an important amino acid that plays a significant role in various bodily functions, including the growth and maintenance of tissues, particularly in the bones, skin, and hair. The lack of lysine in cereal proteins can lead to an imbalanced diet and potentially cause health issues such as growth retardation and weakened immune function.

**Why Each Wrong Option is Incorrect**
**Option A:** Threonine is an essential amino acid found in many plant-based proteins, including cereal proteins, making it an unlikely answer.
**Option B:** Histidine is another essential amino acid, but it is not typically associated with cereal protein deficiency.
**Option C:** Arginine is a semi-essential amino acid, meaning it can be synthesized by the body, and is not typically deficient in cereal proteins.

**Clinical Pearl / High-Yield Fact**
It's essential to note that cereal proteins can be complemented by legume proteins, which are rich in lysine, to form a complete protein that provides all essential amino acids.

**Correct Answer:** D. Lysine