Functional Constipation: This is the most common cause of constipation in infants, often due to changes in diet, dehydration, or lack of fiber. The infant's age and the fact that constipation is a common issue in this developmental stage support this diagnosis.

Dietary Factors: Introduction of new foods or formula can sometimes cause constipation. The infant's diet may be lacking in fiber or too high in constipating foods.
Dehydration: Not enough fluid intake can lead to constipation. In infants, this can happen quickly, especially in warm environments or with inadequate breastfeeding or formula feeding.
Hirschsprung's Disease (partial): While less common, a partial form of Hirschsprung's disease could present with constipation. However, it's typically identified earlier in life.

Intestinal Obstruction: Although rare, conditions like intussusception or volvulus can cause severe constipation and are medical emergencies.
Hirschsprung's Disease (full): The full form of this disease is characterized by the absence of ganglion cells in the distal bowel, leading to severe constipation. It's crucial to diagnose early to prevent complications.
Spinal Cord Anomalies: Conditions affecting the spinal cord, such as spina bifida, can impact bowel function and cause constipation.

Cystic Fibrosis: This genetic disorder can affect the digestive system and cause constipation due to the thick, sticky secretions it produces.
Hypothyroidism: Congenital hypothyroidism can cause constipation among other symptoms. It's essential to consider this, especially if other signs like failure to thrive are present.
Neurological Disorders: Certain neurological conditions can affect bowel habits, though these would be extremely rare as a primary presentation in a 7-month-old with constipation.