## Core Concept
Late metabolic acidosis in newborns, particularly those fed with certain formulas, is often related to the **inability to effectively metabolize certain substances**, leading to an accumulation of acidic metabolites. This condition can arise due to the immaturity of the newborn's metabolic pathways.

## Why the Correct Answer is Right
The correct answer, **D.**, involves formulas with **high chloride content** and **low bicarbonate content**, or those containing **arginine hydrochloride** or **other acidogenic amino acids**. However, a more specific cause is related to the **inability to excrete hydrogen ions** effectively and the **immaturity of renal mechanisms** for acid-base balance. Formulas that are **high in casein** (a protein source) can lead to an increased acid load because casein is **high in sulfur-containing amino acids** (like methionine), which are metabolized to **sulfuric acid**, contributing to metabolic acidosis.

## Why Each Wrong Option is Incorrect
- **Option A:** This option might contain ingredients that are less likely to cause metabolic acidosis or might be more easily metabolized by newborns.
- **Option B:** Similarly, this could have a balanced profile or ingredients that mitigate acidosis.
- **Option C:** This might have a composition that is closer to breast milk or is specifically designed to reduce the risk of metabolic acidosis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **metabolic acidosis in newborns can be prevented or minimized** by ensuring that the formula feeding is **close to the composition of breast milk**, with adequate amounts of **citrate and other base precursors**. Newborns, especially those with **premature kidneys**, are at risk, making the choice of formula critical.

## Correct Answer: D.