A 10 year old boy is having polyuria, polydipsia,laboratory data showed (in mEq/lit) – Na– 154 K– 4.5 HCO3–  
 22 Serum osmolality – 295 Blood urea – 50 Urine specific gravity – 1.005 The likely diagnosis is –
(A) Diabetes insipidus (B) Renal tubular acidosis (C) Barter's syndrome (D) Recurrent UTI
(A) Lets see each option one by one :
artter's syndrome :

is characterized by hypokalemia, metabolic alkalosis and normal to low blood pressure.
It can be easily ruled out as the patient is having normal potassium level. 

Renal tubular acidosis :

is characterized by hyperchloremic metabolic acidosis with a normal serum anion gap.
It is ruled out as this patient is having normal bicarbonate level (Bicarbonate level is decreased in RTA). Recurrent UTI
Symptoms seen are : Fever, dysuria, frequency, urgency, suprapubic pain.
It is ruled out as the above given symptoms are not mentioned in this patient.
Hypernatremia (Na is 154) is not seen in UTI.

iabetes insipidus :

Clinical presentation of hypernatremia, polyuria, polydypsia, thin urine of low specific gravity (1.005) is suggestive of diabetes insipidus.