**Core Concept**
The infant's symptoms and laboratory findings suggest a metabolic disorder affecting renal function and electrolyte balance, leading to acid-base disturbances and electrolyte imbalances.

**Why the Correct Answer is Right**
The infant's presentation of poor weight gain, vomiting, episodic fevers, and chronic constipation, along with laboratory findings of a high urine pH, normal urine anion gap, hyperchloremic metabolic acidosis, and low serum phosphorus and calcium, is consistent with Renal Tubular Acidosis (RTA). RTA, specifically Type 1 or Distal RTA, is characterized by impaired hydrogen ion secretion in the distal tubules, leading to a failure to acidify the urine and resulting in metabolic acidosis. The normal urine anion gap and high urine pH support this diagnosis. Additionally, the low serum phosphorus and calcium levels are consistent with secondary hyperparathyroidism, a common complication of RTA.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not account for the infant's specific laboratory findings, particularly the normal urine anion gap and high urine pH.
**Option B:** This choice is incorrect because it does not explain the infant's metabolic acidosis or the low serum phosphorus and calcium levels.
**Option C:** This choice is incorrect because it does not match the infant's presentation of chronic constipation or the laboratory findings of a high urine pH and normal urine anion gap.

**Clinical Pearl / High-Yield Fact**
Infants with Renal Tubular Acidosis (RTA) may present with a range of symptoms, including growth retardation, failure to thrive, and metabolic acidosis. Early recognition and treatment of RTA are crucial to prevent complications such as secondary hyperparathyroidism and renal osteodystrophy.

**Correct Answer:** C. Distal Renal Tubular Acidosis (RTA)