Pilomatricoma: A common benign skin tumor that can present as a firm lump, often found on the head, neck, or back. It's more common in children and can be mistaken for other conditions due to its firm texture.

Lipoma: Although typically soft, some lipomas can feel firm, especially if they are deep-seated or have undergone fibrosis. They are benign tumors composed of fat tissue.
Epidermoid Cyst: These are benign cysts that can feel firm and are usually found on the skin. They can become inflamed or infected, changing their texture.
Dermoid Cyst: Similar to epidermoid cysts but can contain hair, sweat glands, or other skin structures. They can present as firm lumps under the skin.

Neuroblastoma: A type of cancer that can present as a firm lump in the back, often in the lower back area. It's crucial to consider this diagnosis due to its potential severity and the need for early intervention.
Osteoma: A benign bone tumor, but in rare cases, it could be a sign of a more serious condition if it's found in an unusual location or has specific characteristics.
Spinal Dysraphism: A congenital condition where there's an abnormality in the spine's development, which could present as a lump on the lower back. Early diagnosis is crucial for proper management.

Rhabdomyosarcoma: A soft tissue cancer that could present as a firm lump, though it's more commonly associated with muscles.
Teratoma: A type of tumor that can contain several different types of tissues, including hair, muscle, and bone. It can present as a firm lump and is more commonly found in the lower back or sacrococcygeal area in children.
Congenital Lipoblastoma: A rare benign tumor of fat tissue that can present in infancy or early childhood, sometimes feeling firm due to its location or composition.