An 8–day old breast–fed baby presents with vomiting, poor feeding and loose stools. On examination the heart rate is 190/minute, blood pressure 50/30 mmHg, respiratory rate 72 breaths/minute and capillary refill time of 4 seconds. Investigations show hemoglobin level of 15 g/dl. Na 120 mEq/l, K 6.8 mEq/l, Cl 81 meq/l, bicarbonate 15 mEq/l, urea 30 mg/dl and creatinine 0.6 mg/dl. the most likely diagnosis is –
(A) Congenital adrenal hyperplasia (B) Acute tubular necrosis (C) Congenital hypertrophic pyloric stenosis (D) Galactosemia
(B) Both the serum creatinine and serum urea are abnormal in this neonate. They may be normal for an adult, but for an 8 day old infant they are elevated.
First the serum creatinine level
 
The serum creatinine level is high at birth reflecting the maternal value but it falls rapidly to 0.4 mg/dl by the end of the first week.
According to Nelson, creatinine level in various age groups are
Cord blood     ______ >      0.6 —1.2 mg/di
New born _______ >       0.3 — 1.0 mg/dl
Infant       ______ >                           0.2 - 0.4 mg/di
Child       ______ >                           0.3 - 0.7 mg/di
Adolescent ______ >       0.5 — 1.0 mg/di
Do not get confused by the creatinine level of the newborn i.e., 0.3 — 1.0 mg/dl.
This high level reflects the maternal value and it comes down to 0.2 to 0.4 mg/dl by 5th day
So, Creatinine level of 0.6 mg/dl is abnormally high in an 8 day old infant.
Now, the serum urea level
The serum urea level in cord blood is 21-40 mg/dl, but it falls rapidly to 3-12 mg/dl by the 4th or 5th day.
So urea level of 30 mg/dl is abnormally high in an 8 day old infant.
According to Nelson Serum urea level in various age group
Cord blood   ______ >                     21-40 ing/dl
Premature      ______ >                    3-25 mg/dl
Newborn   ______ >                         3-12 mg/dl
Infant/child _____ >       5-18 ing/d1
Now, the capillary refill time
Capillary refill time is also prolonged here (N ---> < 3 seconds)
Prolonged capillary refill time indicates loss offluid and indicates shock, heart failure, Sympathetic stimulation
In acute renal failure due to fluid loss cappillary refill time is increased
All these findings plus characteristic h/o vomiting, poor feeding and loose stools confirms the diagnosis of acute renal failure (acute tubular necrosis)
Electrolyte abnormalities in ARF --->             1) Hyponatremia,        2) Hyperkalemia,       3) Loss of Bicarbonate 
About other options

In other three options serum urea and creatinine level will be normal.