✅ Model Answer:
• Diagnosis: Pseudohypoparathyroidism type 1a (Albright hereditary osteodystrophy) – PTH resistance due to GNAS mutation. Features: low calcium, high phosphorus, elevated PTH, normal vitamin D, low urinary calcium (inadequate PTH response).
• Any other test: GNAS gene sequencing; Ellsworth-Howard test (PTH infusion) – blunted urinary cAMP and phosphate response; check TSH (TSH resistance common); X-ray of hands (brachydactyly, short metacarpals); subcutaneous calcifications.
• What to do next: Start oral elemental calcium 25-50 mg/kg/day + calcitriol 0.01-0.1 mcg/kg/day. Goal low-normal calcium (8-8.5 mg/dL) to avoid hypercalciuria.
• Follow-up plan: Monitor serum calcium, phosphorus, PTH; urine calcium/creatinine ratio (goal <0.2-0.3); check thyroid function (TSH resistance) – if hypothyroidism, start levothyroxine standard dose. Lifelong therapy. Genetic counseling (autosomal dominant with maternal imprinting).

✅ Model Answer:
• Gene: GNAS (guanine nucleotide-binding protein, alpha-stimulating activity polypeptide 1) located on chromosome 20q13.3.
• Inheritance: Autosomal dominant with maternal imprinting (paternal inheritance causes pseudopseudohypoparathyroidism without hormone resistance).
• Mutation: Inactivating mutations in GNAS (exon 1, missense, nonsense, frameshift, splice-site).
• Pathophysiology: Defective Gsα protein → impaired PTH receptor signaling → PTH resistance in the kidney and bone → ↓ urinary cAMP, ↓ phosphate excretion, ↓ 1α-hydroxylase activation → ↓ calcium.
• Other hormone resistance: TSH resistance (mild hypothyroidism), gonadotropin resistance (delayed puberty), GHRH resistance (growth hormone deficiency).

✅ Model Answer:
• Classic features (AHO phenotype):
- Short stature: Growth failure.
- Round face: Moon facies.
- Brachydactyly: Short 4th and 5th metacarpals (most characteristic), short metatarsals.
- Obesity: Truncal obesity.
- Subcutaneous ossifications: Calcified nodules in the skin.
- Intellectual disability: Mild to moderate.
- Dental anomalies: Enamel hypoplasia, delayed dentition.
• Hormone resistance features:
- Hypocalcemia, hyperphosphatemia.
- TSH resistance: Mild hypothyroidism.
- Gonadotropin resistance: Delayed puberty (less common).
- Growth hormone resistance.

✅ Model Answer:
• Ellsworth-Howard test: A functional test to assess PTH responsiveness.
• Procedure:
- Baseline urine is collected for cyclic AMP (cAMP) and phosphate.
- IV or IM injection of synthetic PTH (e.g., teriparatide 200-400 IU).
- Urine is collected for 1-2 hours post-injection.
• Interpretation:
- Normal response: Urinary cAMP increases ≥10-fold, phosphate excretion increases (phosphaturic response).
- Pseudohypoparathyroidism: Blunted or absent urinary cAMP and phosphate response (PTH resistance).
- Hypoparathyroidism: Normal cAMP and phosphate response (but PTH is low).
• Clinical use: Differentiates PHP from hypoparathyroidism (rarely used now, replaced by genetic testing).
• Alternative: Genetic testing for GNAS is now the preferred diagnostic tool.

✅ Model Answer:
• Calcium supplementation:
- Elemental calcium: 25-50 mg/kg/day in 2-3 divided doses (higher doses may be needed).
- Goal: Maintain serum calcium at low-normal (8-8.5 mg/dL) to avoid hypercalciuria.
• Calcitriol (1,25-dihydroxyvitamin D):
- Dose: 0.01-0.1 mcg/kg/day (usually 0.25-1 mcg/day).
- Necessary: Because PTH resistance impairs renal 1α-hydroxylase activation.
• Thyroid hormone replacement:
- If TSH resistance causes hypothyroidism: start levothyroxine (standard dose based on age/weight).
• Monitoring:
- Serum calcium, phosphorus, PTH: Every 3-6 months.
- Urine calcium/creatinine ratio: Goal <0.2-0.3 (to avoid nephrocalcinosis).
- TSH, free T4: To monitor thyroid function.
- Growth monitoring.

✅ Model Answer:
• Hypocalcemic complications:
- Seizures: Tonic-clonic or focal.
- Tetany: Carpopedal spasm, perioral tingling, Chvostek sign, Trousseau sign.
- Prolonged QT interval: Risk of arrhythmias.
- Basal ganglia calcification: May cause movement disorders or Parkinsonism.
- Cataracts.
• Hyperphosphatemia:
- Soft tissue calcifications: Subcutaneous, renal, vascular.
- Nephrocalcinosis: If calcium-phosphate product is elevated.
• Hormone resistance complications:
- Hypothyroidism: If TSH resistance is significant.
- Delayed puberty: Due to gonadotropin resistance.
- Growth failure.
• Obesity: Increased risk of metabolic syndrome.
• Alloimmunization: If on chronic calcium (not relevant).

✅ Model Answer:
• Prognosis:
- Good with appropriate calcium/calcitriol therapy.
- Life expectancy: Normal.
- Morbidity: Mainly from hypocalcemic seizures, basal ganglia calcification, and obesity.
- Intellectual disability: Usually mild to moderate, non-progressive.
- Brachydactyly: Persistent, no treatment needed.
• Long-term follow-up:
- Calcium/calcitriol: Lifelong therapy.
- Monitor calcium, phosphorus, PTH: Every 3-6 months.
- Urine calcium/creatinine: To avoid nephrocalcinosis.
- Thyroid function: Check TSH, free T4 annually.
- Growth and development monitoring.
- Genetic counseling: Autosomal dominant with maternal imprinting (50% risk of transmission).
- Obesity management: Diet and exercise.

✅ Model Answer:
• Both are caused by GNAS mutations.
• Pseudohypoparathyroidism (PHP type 1a):
- Inheritance: Maternal transmission of GNAS mutation.
- Features: AHO phenotype + hormone resistance (PTH resistance → hypocalcemia, hyperphosphatemia, elevated PTH; also TSH, gonadotropin resistance).
- Lab: ↓Ca, ↑P, ↑PTH.
• Pseudopseudohypoparathyroidism (PPHP):
- Inheritance: Paternal transmission of GNAS mutation.
- Features: AHO phenotype ONLY (short stature, round face, brachydactyly) – NO hormone resistance.
- Lab: Normal calcium, phosphorus, PTH (no biochemical abnormalities).
• Genetic basis: GNAS is imprinted – maternal allele is expressed in the kidney, bone, and thyroid; paternal allele is silenced in these tissues.
• Clinical significance: Both have the same physical features, but only maternal inheritance causes hormone resistance.