A newborn baby is noted to have a left unilateral cleft lip. There are no abnormalities of the baby's palate. Which of the following developmental defects accounts for this occurrence?

In the formation of the upper lip, the right and left medial nasal prominences merge to form the philtrum of the upper lip. The lateral maxillary prominence then merges with the merged medial nasal prominences. Failure of this merger to occur results in a unilateral cleft lip.Failure of a lateral palatine process to fuse with the median palatine process, which is synonymous with a failure of the primary palate to fuse with the secondary palate, results in a primary cleft palate. Primary clefts of the palate are found anterior to the incisive foramen of the palate. Primary cleft palates may occur along with cleft lips but are the result of a separate developmental defect.Failure of the right and left medial nasal prominences to merge results in a median cleft lip. This is a rare anomaly. Normally, the right and left medial nasal prominences merge into a single prominence which forms the philtrum of the lip. Ref: Losee J.E., Gimbel M., Rubin J., Wallace C.G., Wei F. (2010). Chapter 45. Plastic and Reconstructive Surgery. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.