Neuroblastoma: This is the most likely diagnosis given the age of the patient, the presence of a painless abdominal mass that extends to the midline, and the increase in catecholamines. Neuroblastoma is a common extracranial solid tumor in children, often presenting with an abdominal mass, and it can secrete catecholamines, leading to elevated levels.

Nephroblastoma (Wilms Tumor): Although less common than neuroblastoma in infants, Wilms tumor is a consideration for any child presenting with an abdominal mass. It typically presents as a solitary, well-defined mass in the kidney and can extend to the midline. However, it is less likely to cause an increase in catecholamines compared to neuroblastoma.
Lymphoma: While lymphoma can present with an abdominal mass, it is less common in infants and typically would not be the first consideration for a painless abdominal mass extending to the midline with increased catecholamines.

Pheochromocytoma: Although rare in infants, pheochromocytoma can cause an increase in catecholamines and present as an abdominal mass. It is crucial to consider this diagnosis due to its potential for severe hypertension and other complications if not promptly treated.
Ganglioneuroma: This is a rare, benign tumor that can also secrete catecholamines. It is less aggressive than neuroblastoma but can still present with an abdominal mass.

Hepatoblastoma: A rare liver cancer that can present as an abdominal mass in infants but is less likely to cause an increase in catecholamines.
Rhabdoid Tumor: A rare, aggressive tumor that can occur in the kidney or other soft tissues, presenting as an abdominal mass, but it is not typically associated with increased catecholamine levels.
Other rare abdominal tumors: Such as teratomas or sarcomas, which can present with a mass but are less likely to have the specific combination of features described (midline extension and increased catecholamines).