A 5-day-old child presents with intractable seizures. He had rashes all over the body. Blood examination showed hyperammonemia and lactic acidosis. The probable diagnosis is –
Organic aciduria (organic acidemia)

Organic aciduria is a term used to classify a group of metabolic disorders which disrupt normal amino acid metabolism, particularly branched-chain amino acids (isoleucine, leucine, valise) causing a build of acids which are usually not present.
The main types of organic acidemia are : -

i) Methylnzalonic acidemia    
ii) Propionic acidenzia       
iii) Isoavleric acidemia
iv) Maple syrup urine disease
vi) Giutaric acidemia           
v) Combined (multiple) carboxylase deficiency
Clinical manifestations

Neurological damage → Mental retardation, Seizures, Lethargy, Vomiting.
Developmental delay               
Hypoglycemia                           
Ketoacidosis        
Malnutrition     
Hypotonia     
Hyperammonemia     
Poor feeding and slow growth  
Metabolic acidosis (Lactic acidosis)  
Rash, alopecia and ataxia in some organic aciduria.            

Mitochondrial encephalopathy (MELAS)

Encephalopathy
Lactic acidosis
Stroke-like episodes → Blindness, paralysis, vomiting.

Disorders of the urea cycle

Ammonia is mostly derived from the catabolism of amino acid and is converted to an amino group in urea by enzymes of the urea cycle.
So, defects in enzymes of urea cycle cause →

i) Hyperammonemia           
ii) Encephalopathy     
iii) Vomiting
Organic aciduria
Mitochondrial encephalopathy with lactic aciduria
Phenylketonuria
Urea cycle enzyme deficiency