A 10 years old boy is having polyuria, polydipsia. Laboratory data showed (in meq/L): Na+ – 154, K+ – 4.5, HCO3- 22, S. Osmolality – 295, B. Urea – 50, Urine specific gravity – 1.005 What is the diagnosis?
First, the key symptoms are polyuria and polydipsia, which are classic signs of diabetes insipidus (DI) or diabetes mellitus. But the urine specific gravity is low, which suggests the kidneys are not concentrating urine properly. In diabetes mellitus, you'd expect high glucose in urine, but the question doesn't mention that.
Hypernatremia here is due to dehydration from excessive water loss through urine. The low urine specific gravity points towards central DI (inadequate ADH) rather than nephrogenic DI (kidneys not responding to ADH), because in nephrogenic DI, the urine would be more variable in concentration depending on the cause. However, in central DI, the lack of ADH leads to dilute urine regardless.
The elevated blood urea (50) could be due to dehydration causing pre-renal azotemia. The serum osmolality is high because of the hypernatremia. So the main diagnosis here is central DI. The options might include central DI, nephrogenic DI, diabetes mellitus, and other causes like SIADH. But SIADH would have low serum osmolality and high urine osmolality, which isn't the case here.
The wrong options would be other conditions that can present with similar symptoms but don't fit the lab findings. For example, diabetes mellitus would have hyperglycemia and glycosuria, which isn't mentioned. Nephrogenic DI might have a different urine osmolality profile. So the correct answer is central DI.
**Core Concept**
This question tests the diagnosis of hypernatremia with polyuria and dilute urine, focusing on differential diagnoses like diabetes insipidus (DI). Key lab findings include elevated sodium, low urine specific gravity, and hyperosmolality.
**Why the Correct Answer is Right**
The clinical presentation of polyuria, polydipsia, and hypernatremia (Na⁺ 154 meq/L) with dilute urine (specific gravity 1.005) strongly suggests **central diabetes insipidus (DI)**. ADH deficiency impairs renal water reabsorption in the collecting ducts, leading to excessive water loss and compensatory thirst. The low urine specific gravity confirms the kidneys are not concentrating urine despite hypernatremia. Elevated blood urea (50) reflects dehydration-induced pre-renal azotemia.
**Why Each Wrong Option is Incorrect**
**Option A:** *Nephrogenic DI* is incorrect because it results from renal resistance to ADH, not deficiency. Urine osmolality would often be inappropriately high in response to exogenous ADH.
**Option B:** *Diabetes mellitus* is incorrect as it presents with