Gitelman Syndrome: This is a genetic disorder characterized by low plasma renin activity, normal or low serum aldosterone, hypokalemia, hypomagnesemia, and hypophosphatemia. The presence of loose motions (diarrhea) can also be seen due to the imbalance of electrolytes. The combination of these symptoms aligns closely with Gitelman Syndrome, making it a strong candidate for the single most likely diagnosis.

Bartter Syndrome: Similar to Gitelman Syndrome, Bartter Syndrome is a genetic disorder affecting the kidneys' ability to reabsorb electrolytes, leading to hypokalemia, hypomagnesemia, and often hypophosphatemia. However, it typically presents at a younger age and with more pronounced renal abnormalities.
Primary Aldosteronism with Renal Resistance to Aldosterone: Although serum aldosterone is normal, some forms of primary aldosteronism can lead to renal resistance, resulting in hypokalemia and other electrolyte imbalances. The loose motions could be secondary to the electrolyte disturbances.
Liddle Syndrome: This rare genetic disorder leads to excessive sodium reabsorption and potassium secretion in the kidneys, resulting in hypokalemia. However, it typically presents with hypertension, which is not mentioned in the scenario.

Adrenal Insufficiency: Although less likely given the normal serum aldosterone, adrenal insufficiency can present with hypotension, electrolyte imbalances, and gastrointestinal symptoms like diarrhea. Missing this diagnosis could be fatal due to the risk of adrenal crisis.
Medication-Induced Hypokalemia: Certain medications (e.g., diuretics, laxatives) can cause significant electrolyte imbalances, including hypokalemia, hypophosphatemia, and hypomagnesemia. The presence of loose motions could be a clue to laxative abuse or misuse.

East Syndrome (EAST Syndrome): A rare genetic disorder characterized by epilepsy, ataxia, sensorineural deafness, and tubulopathy. The tubulopathy can lead to electrolyte imbalances similar to those described.
Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis: This genetic disorder affects the reabsorption of magnesium and calcium in the kidneys, leading to hypomagnesemia and other electrolyte disturbances. It is a rare condition but should be considered in the differential diagnosis of persistent hypomagnesemia.