## Core Concept
The question tests knowledge of the nutritional content of common food grains, specifically amino acid deficiencies in wheat and maize. Amino acids are the building blocks of proteins, and certain foods are known to be deficient in specific amino acids, which can impact overall protein quality.

## Why the Correct Answer is Right
Lysine is an essential amino acid that the human body cannot synthesize on its own and must obtain from dietary sources. Both wheat and maize are known to be relatively low in lysine. Wheat contains a high amount of the amino acids glutamic acid, proline, and aspartic acid but is relatively low in lysine. Similarly, maize (corn) is also low in lysine and tryptophan. This deficiency is significant because lysine is crucial for growth and development.

## Why Each Wrong Option is Incorrect
- **Option A: Tryptophan** - While it's true that maize is low in tryptophan, wheat is not particularly noted for a deficiency in tryptophan compared to its deficiency in lysine.
- **Option B: Methionine** - Methionine is an essential amino acid but is not notably deficient in both wheat and maize; if anything, these grains can have adequate or even relatively higher amounts compared to lysine.
- **Option C: Threonine** - Threonine is another essential amino acid, but both wheat and maize do not stand out for being particularly deficient in threonine compared to lysine.

## Clinical Pearl / High-Yield Fact
A key point to remember is that diets predominantly based on wheat or maize can lead to a deficiency in lysine if not properly supplemented with lysine-rich foods or if the diet does not include a variety of protein sources. This is especially relevant in areas where these grains are staple foods.

## Correct Answer Line
**Correct Answer: D. Lysine.**