بنك الأسئلة المجّاني للاختبارات الوطنيّة SMLE و SNLE

The boy in the vignette has symptoms and signs consistent with hypocalcemia. Trousseau sign occurs when a blood pressure cuff is placed around the arm and inflated to a pressure greater than the systolic blood pressure and held in place. In the absence of blood flow, the patient's hypocalcemia and subsequent neuromuscular irritability will induce spasm of the hand and forearm muscles. Chvostek sign is an abnormal reaction to the stimulation of the facial nerve when hypocalcemia is present. When the facial nerve is tapped at the angle of the jaw, the facial muscles on the same side of the face will contract momentarily (typically a twitch of the nose or lips). The most common reason for hypocalcemia to occur is vitamin D deficiency. Natural sunlight is the major source of vitamin D for children and adolescents, therefore inadequate exposure during the winter months may lead to low vitamin D levels. People with naturally dark skin tone require at least 3 to 5 times longer sun exposure to produce the same levels of vitamin D as those with light skin tone. Thus, for the adolescent in the vignette, vitamin D deficiency would be the most likely cause of hypocalcemia. The laboratory findings in vitamin D deficiency are summarized in Item C239. Early in the development of vitamin D deficiency, serum parathyroid hormone (PTH) levels increase, which is followed later by increased alkaline phosphatase levels. Calcium and phosphorus may initially be normal. 25-hydroxyvitamin D, with a circulating half-life of 2 to 3 weeks, is the major circulating form and is the best indicator of vitamin D status. Although 1,25-dihydroxyvitamin D is traditionally taught to be the “active form” of the hormone, its measurement does not reflect vitamin D status. 1,25-dihydroxyvitamin D has a circulating half-life of approximately 4 hours. It circulates at 1,000 times lower concentration than 25-hydroxyvitamin, and the blood level is tightly regulated by serum levels of PTH, calcium, and phosphate. Serum 1,25-dihydroxyvitamin D is frequently either normal or even elevated in those with vitamin D deficiency because of secondary hyperparathyroidism. Although the patient in this vignette has muscle aches, measurement of creatine kinase is not relevant when considered along with the patient’s history and physical examination. Levels of PTH and alkaline phosphatase will both be elevated in the presence of hypocalcemia, however, they can be elevated due to any cause of increased bone turnover, therefore measurement will not identify the specific diagnosis. The 2006 American Academy of Pediatrics guidelines on calcium and vitamin D intake include recommended sources of nutritional intake to optimize bone health. Guidelines from the Pediatric Endocrine Society also recognize the need to optimize nutritional intake, given the increased use of sunscreen during the summer and limited sun exposure during winter months #### PREP Pearls * Measurement of 25-hydroxyvitamin D (the storage form of vitamin D) is the definitive test for diagnosing vitamin D deficiency. * Vitamin D deficiency is the most common cause of low calcium levels in adolescence, especially during the winter months when sun exposure is limited.