Okay, let's tackle this question. The infant is crying a lot, has been fed well but still dehydrated, passing a lot of urine. The urine has low specific gravity and no protein. The key points here are the polyuria and dehydration despite adequate feeding, and the low specific gravity.

First, I need to think about conditions that cause polyuria and dehydration in infants. The most common ones would be diabetes insipidus (DI) and diabetes mellitus (DM). But in infants, DM is less likely unless there's a family history or other signs like glucose in urine. Since the question mentions no proteinuria, DM is probably out because hyperglycemia leads to glucosuria and osmotic diuresis. But in this case, there's no protein, so maybe not DM.

Diabetes insipidus is more likely here. DI can be central (lack of ADH) or nephrogenic (kidneys don't respond to ADH). The low specific gravity in DI is because the kidneys can't concentrate urine. In central DI, the urine would be very dilute, and the infant would be thirsty and dehydrated. Nephrogenic DI would also present similarly but might have other causes like electrolyte imbalances or genetic factors.

Other options could be conditions like renal tubular acidosis, but that usually presents with metabolic acidosis and other electrolyte issues. Or maybe a UTI, but UTIs in infants are more likely to have fever, poor feeding, and possibly proteinuria if there's inflammation. The absence of proteinuria here makes UTI less likely.

So the most likely diagnosis is diabetes insipidus. The low specific gravity is a key clue because it indicates the kidneys aren't concentrating the urine properly, which is a hallmark of DI. The clinical scenario fits with the infant being dehydrated despite polyuria, which is a classic presentation of DI.

**Core Concept**
The clinical scenario describes polyuria, dehydration, and low urine specific gravity in an infant, pointing to impaired renal concentrating ability. This is characteristic of **diabetes insipidus (DI)**, a disorder of **antidiuretic hormone (ADH)** deficiency or resistance.

**Why the Correct Answer is Right**
In DI, the kidneys fail to concentrate urine due to **ADH deficiency (central DI)** or **renal insensitivity to ADH (nephrogenic DI)**. The low urine specific gravity (1004) reflects dilute urine, and excessive water loss leads to dehydration despite adequate oral intake. Infants with DI often present with **polydipsia (excessive thirst)** and **polyuria**, but may not communicate thirst effectively, resulting in dehydration and irritability.

**Why Each Wrong Option is Incorrect**
**Option A:** *Diabetes mellitus* would cause **glucosuria** and **osmotic diuresis**, but **no proteinuria** is noted here.
**Option B:** *Renal tubular acidosis* typically presents with **metabolic acidosis** and **electrolyte abnormalities**, not isolated polyuria.
**Option C:** *Urinary tract infection* (UTI) in infants may cause fever, poor feeding, and **proteinuria**, none of which are present.

**Clinical Pearl / High-Yield Fact**
Remember the **"DI triangle

✓ Correct Answer: B. Diabetes insipidus