**Core Concept**
The infant's presentation of excessive crying, polyuria, and recurrent dehydration despite adequate feeding suggests a disorder of water balance regulation. The specific gravity of the urine, which is close to 1.000, indicates that the infant is producing very dilute urine, suggesting an inability to concentrate urine.

**Why the Correct Answer is Right**
The infant's symptoms are consistent with diabetes insipidus (DI), a condition characterized by the inability to concentrate urine due to a deficiency of antidiuretic hormone (ADH) or its action. In central DI, the pituitary gland fails to produce ADH, leading to excessive water loss and polyuria. The normal specific gravity of the urine and the absence of proteinuria help to rule out other causes of polyuria, such as diabetes mellitus or renal disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the infant's specific symptoms of polyuria and dehydration. Diarrhea may contribute to fluid loss, but it does not explain the infant's inability to concentrate urine.
**Option B:** This option is incorrect because it is not a specific diagnosis that explains the infant's symptoms. "Reflux" is a nonspecific term that can refer to various conditions, including gastroesophageal reflux disease (GERD). However, GERD would not cause polyuria or dehydration.
**Option C:** This option is incorrect because it is not a diagnosis that explains the infant's symptoms. "Infection" is a nonspecific term that can refer to various conditions, including urinary tract infections (UTIs). However, UTIs would typically be associated with proteinuria, hematuria, or other urinary abnormalities.

**Clinical Pearl / High-Yield Fact**
Diabetes insipidus is a classic cause of polyuria and dehydration in infants, and it is essential to consider this diagnosis in any infant presenting with these symptoms. The measurement of urine specific gravity and the assessment of ADH levels can help diagnose DI.

**Correct Answer:** C.