Evaluation of an infant reveals hypocalcemia due to a lack of parathyroid hormone. On x-ray, the thymic shadow is absent. Which of the following embryonic structures is involved
**Core Concept**
The question is testing the understanding of the embryological development of the parathyroid glands and their relationship with the thymus. The parathyroid glands and the thymus develop from the third pharyngeal pouch and the third pharyngeal cleft, respectively.
**Why the Correct Answer is Right**
In the case of DiGeorge syndrome (also known as 22q11.2 deletion syndrome), the third and fourth pharyngeal pouches fail to develop, leading to hypoplasia or aplasia of the parathyroid glands and the thymus. This results in hypocalcemia due to the lack of parathyroid hormone and immunodeficiency due to the absence of the thymus. The thymic shadow is absent on x-ray because the thymus is not developed.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the embryological development of the parathyroid glands and the thymus. The first pharyngeal pouch gives rise to the tonsil, and the second pharyngeal pouch gives rise to the palatine tonsil and the middle ear cavity.
**Option B:** This option is incorrect because it is a structure that develops from the first pharyngeal pouch, not the third pharyngeal pouch.
**Option D:** This option is incorrect because it is a structure that develops from the second pharyngeal pouch, not the third pharyngeal pouch.
**Clinical Pearl / High-Yield Fact**
The DiGeorge syndrome is characterized by a triad of hypocalcemia, congenital heart disease, and cleft palate. It is also associated with an increased risk of autoimmune disorders and immunodeficiency.
**Correct Answer: C. Third pharyngeal pouch**