**Core Concept**
Sodium loss in children can occur through various mechanisms, including renal losses due to excessive sodium excretion, gastrointestinal losses due to diarrhea or vomiting, and transcellular shifts. The body tightly regulates sodium levels through the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system.

**Why the Correct Answer is Right**
The maximum loss of sodium in a child occurs through the gastrointestinal tract due to diarrhea. Diarrhea causes a massive loss of sodium, chloride, and water from the gut due to the increased permeability of the intestinal mucosa. This is because the intestinal mucosa secretes more chloride ions into the lumen, which in turn causes a massive loss of sodium ions through the paracellular pathway. This process is mediated by the cystic fibrosis transmembrane conductance regulator (CFTR) channel, which is responsible for the secretion of chloride ions into the lumen.

**Why Each Wrong Option is Incorrect**
**Option A:** Renal losses due to excessive sodium excretion can occur in conditions such as primary aldosteronism, but this is not the maximum loss of sodium in a child.
**Option B:** Transcellular shifts of sodium can occur in conditions such as adrenal insufficiency or hypokalemia, but this is not the maximum loss of sodium in a child.
**Option C:** (This option is missing)

**Clinical Pearl / High-Yield Fact**
In children, the maximum loss of sodium occurs through the gastrointestinal tract due to diarrhea, and this can lead to severe dehydration and hypovolemic shock if not promptly treated.

**Correct Answer:** D.