A 12 year old boy presents with hemetemesis, malena and mild splenomegaly. There is no obvious jaundice or ascitis. The most likely diagnosis is –
Information in question are:


Patient is 12 years male
Hematemesis, melena and mild splenomegaly —k signs of portal hypertension
No Jaundice or Ascitis


So, the boy in question has portal hypertension.
Three most common causes of portal hypertension are:


Extrahepatic portal vein obstruction (EHPVO)
Non-cirrhotic portal fibrosis (NCPF)
Cirrhosis.


In cirrhosis, jaundice & ascitis are prominent. So, this option is ruled out.
Now we are left with EHPVO and NCPF. Both these causes non-cirrhotic portal hypertension, i.e. portal hypertension without signs of liver dysfunction.
Both these have similar clinical picture except for the age of presentation.
EHPVO is usually' seen in young male children (1st and 2nd decade) while NCPF is a disease of adults (25-35 years).
So, best answer here is EHPVO.
EHPVO
NCPF
Cirrhosis
Malaria with DIC