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Zorluk: OrtaEndocrine and Metabolic Mechanisms

A 22-year-old female presents to the outpatient clinic reporting intermittent muscle cramping and tingling in her fingertips over the past several months. She has a history of mild intellectual disability but currently takes no medications. Vital signs are within normal limits. Physical examination reveals short stature, a round face, and shortened fourth and fifth metacarpals on both hands. Tapping the facial nerve anterior to the earlobe elicits twitching of the ipsilateral facial muscles. Laboratory studies show:

Laboratory TestResultReference Range
Serum calcium7.2 mg/dL8.4-10.2 mg/dL
Serum phosphorus5.6 mg/dL2.5-4.5 mg/dL
Intact parathyroid hormone350 pg/mL15-65 pg/mL
25-hydroxyvitamin D32 ng/mL20-50 ng/mL
Serum creatinine0.8 mg/dL0.6-1.1 mg/dL

Which of the following cellular mechanisms best explains this patient's clinical and laboratory findings?

  1. Impaired stimulatory G-protein subunit signalingCevap
  2. B
    Inactivating mutation of the calcium-sensing receptor
  3. C
    Autoimmune destruction of parathyroid chief cells
  4. D
    Decreased renal synthesis of 1,25-dihydroxycholecalciferol
  5. E
    Cytochrome P450-mediated increased clearance of cholecalciferol

Cevap

Impaired stimulatory G-protein subunit signaling
The correct answer accurately identifies the molecular pathophysiology of pseudohypoparathyroidism type 1a (PHP-1a). This patient's presentation of short stature, round facies, mild intellectual disability, and shortened 4th and 5th metacarpals is pathognomonic for Albright hereditary osteodystrophy. The laboratory findings of hypocalcemia, hyperphosphatemia, and a markedly elevated PTH level—in the presence of normal renal function and normal vitamin D stores—indicate target-organ resistance to parathyroid hormone. The PTH receptor is a G-protein coupled receptor that normally utilizes a stimulatory G-protein alpha subunit (GαsG\alpha_s) to activate adenylate cyclase. PHP-1a is caused by a maternally inherited inactivating mutation in the GNAS1 gene, which encodes this GαsG\alpha_s subunit, leading to impaired intracellular signaling.

Adım Adım Çözüm

1
Identify the clinical syndrome based on the physical examination findings.
The patient has short stature, a round face, and shortened 4th and 5th metacarpals. These are the classic features of Albright hereditary osteodystrophy (AHO). The positive Chvostek sign indicates neuromuscular irritability due to hypocalcemia.
Recognizing the phenotypic presentation narrows the differential diagnosis for hypocalcemia and directs interpretation of the laboratory results.
2
Analyze the laboratory data to determine the functional endocrine state.
The patient has hypocalcemia and hyperphosphatemia despite a markedly elevated intact parathyroid hormone (PTH) level. Renal function and vitamin D stores are normal.
The combination of elevated PTH with hypocalcemia in the setting of normal renal function establishes the diagnosis of target-organ resistance to PTH (pseudohypoparathyroidism).
3
Correlate the clinical diagnosis with its underlying molecular mechanism.
The correct underlying mechanism is an inactivating mutation in the GNAS1 gene, which leads to a defective stimulatory G-protein alpha subunit (GαsG\alpha_s).
PTH acts on target tissues (kidney and bone) via a G-protein coupled receptor. The defective GαsG\alpha_s subunit prevents the generation of cAMP, causing end-organ resistance characteristic of pseudohypoparathyroidism type 1a.

Anahtar Kavram

Pseudohypoparathyroidism type 1a is characterized by end-organ resistance to PTH due to a defective stimulatory G-protein alpha subunit, presenting with hypocalcemia, hyperphosphatemia, elevated PTH, and the Albright hereditary osteodystrophy phenotype.
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Endocrine and Metabolic Mechanisms

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