Pyloric Stenosis: This condition is characterized by the thickening of the pylorus muscle, leading to gastric outlet obstruction. It typically presents with projectile vomiting in infants around 1-5 months of age, making it the most likely diagnosis given the age and symptom.

Gastroesophageal Reflux Disease (GERD): Although GERD more commonly presents with regurgitation rather than projectile vomiting, it's a common condition in infants that can cause significant discomfort and vomiting.
Infectious Gastroenteritis: Viral or bacterial infections can cause vomiting in infants, though the vomiting may not always be projectile.
Overfeeding or Incorrect Feeding Technique: Overfeeding or using an incorrect feeding technique can lead to vomiting in infants.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply. It's a medical emergency that can present with vomiting, though it's more commonly associated with abdominal pain and bloody stools.
Volvolus or Malrotation: These are intestinal obstructions that can cause severe vomiting and are life-threatening if not promptly treated.
Hypertrophic Cardiomyopathy: Though rare, this condition can lead to vomiting due to poor cardiac output and should be considered, especially if there are other signs of heart failure.

Congenital Anomalies (e.g., Duodenal Atresia, Tracheoesophageal Fistula): These conditions are present at birth and can cause vomiting, among other symptoms. They are less common but important to consider in the differential diagnosis.
Metabolic Disorders: Certain metabolic disorders can present with vomiting in infancy, though they are rare and often accompanied by other systemic signs.
Increased Intracranial Pressure: Conditions that increase intracranial pressure, such as hydrocephalus or brain tumors, can cause vomiting, though this would be extremely rare in a 1-month-old infant without other significant neurological signs.