A 67-year-old male presents to his primary care physician complaining of left hip pain for the past six months. He denies any trauma or recent falls. He is accompanied by his wife who reports that he has experienced progressive hearing loss over the same time period. The patient has also noticed that he is no longer able to fit into his favorite hat even though it previously fit well. A radiograph of the patient’s pelvis is shown. Which of the following laboratory abnormalities is most likely to be found in this patient?

Elevated serum alkaline phosphatase

Decreased serum alkaline phosphatase

Elevated serum parathyroid hormone

A 6-year-old boy is brought to the physician for a well-child examination. His mother has noticed he frequently falls while running. He was born at term and pregnancy was uncomplicated. He has a seizure disorder treated with phenytoin. He is at the 20th percentile for height and at 30th percentile for weight. Vital signs are within normal limits. Examination shows decreased muscle strength in the lower extremities. There is a deep groove below the costal margins bilaterally. An x-ray of the lower extremities is shown. Which of the following is the most likely cause of these findings?

Proximal tibial growth plate disruption

A 67-year-old Caucasian female presents to her primary care physician after a screening DEXA scan reveals a T-score of -3.0. Laboratory work-up reveals normal serum calcium, phosphate, vitamin D, and PTH levels. She smokes 1-2 cigarettes per day. Which of the following measures would have reduced this patient's risk of developing osteoporosis?

Reduced physical activity to decrease the chance of a fall

Initiating a swimming exercise program three days per week

Calcium and vitamin D supplementation

A 6-month-old girl is brought to the physician for a well-child examination. She was born at 37 weeks' gestation. Pregnancy and the neonatal period were uncomplicated. The infant was exclusively breastfed and received vitamin D supplementation. She can sit unsupported and can transfer objects from one hand to the other. She babbles and is uncomfortable around strangers. She is at 40th percentile for length and at 35th percentile for weight. Vital signs are within normal limits. Physical examination shows no abnormalities. In addition to continuing breastfeeding, which of the following is the most appropriate recommendation at this time?

Introduce solid foods and continue vitamin D

Introduce solid foods and add vitamin C

Introduce solid foods and cow milk

A 3-month-old African American boy presents to his pediatrician’s office for his routine well visit. He was born full-term from an uncomplicated vaginal delivery. He is exclusively breastfeeding and not receiving any medications or supplements. Today, his parents report no issues or concerns with their child. He is lifting his head for brief periods and smiling. He has received only 2 hepatitis B vaccines. Which of the following is the correct advice for this patient’s parents?

He should start vitamin D supplementation.

He should have his serum lead level checked to screen for lead intoxication.

He needs a 3rd hepatitis B vaccine.

A 35-year-old female presents to her primary care physician complaining of right upper quadrant pain over the last 6 months. Pain is worst after eating and feels like intermittent squeezing. She also admits to lighter colored stools and a feeling of itchiness on her skin. Physical exam demonstrates a positive Murphy's sign. The vitamin level least likely to be affected by this condition is associated with which of the following deficiency syndromes?

Increased prothrombin time and easy bleeding

A child presents with rachitic changes in the limbs that are not responding to Vitamin D supplementation. Investigations reveal the following results: - Calcium: $9.5 \mathrm{mg} / \mathrm{dl}$ - Phosphorus: $1.6 \mathrm{mg} / \mathrm{dl}$ - Alkaline phosphatase (ALP): 814 IU - Serum PTH: $24.2 \mathrm{pg} / \mathrm{ml}$ - Serum electrolytes, creatinine, and blood gases: Normal. What is the most likely diagnosis?

Vitamin D-dependent rickets type 2

Vitamin D-dependent rickets type 1

Vitamin D deficiency and rickets for USMLE Step 2 CK: High-Yield Pediatrics Lessons

Vitamin D Metabolism - Sunshine to Super-Hormone

Sources: Endogenous synthesis in skin via UVB on 7-dehydrocholesterol (forms D3). Dietary intake includes Ergocalciferol (D2) from plants and Cholecalciferol (D3) from animal sources.
Activation Pathway: A two-step hydroxylation process.

Vitamin D synthesis, activation, and metabolism pathways

⭐ 25-hydroxyvitamin D (Calcidiol) is the major circulating form and best indicator of vitamin D status. The rate-limiting step, 1α-hydroxylase in the kidney, is tightly regulated by PTH.

Clinical Features - Signs of Soft Bones

Skull
- Craniotabes: Ping-pong ball sensation (earliest sign).
- Frontal bossing & delayed fontanelle closure.
Thorax
- Rachitic rosary: Palpable beading at costochondral junctions.
- Harrison's sulcus: Horizontal groove along the lower border of the chest.
- Pectus carinatum (pigeon chest).
Limbs
- Enlargement of wrists and ankles.
- Bowing of legs (genu varum) or knock-knees (genu valgum).

Clinical features of rickets

⭐ Craniotabes is the earliest detectable bony sign of rickets.

Diagnosis - Labs & X-Rays

Biochemistry:
- Serum 25(OH)D: ↓ (Deficiency < 20 ng/mL)
- Serum Alkaline Phosphatase (ALP): ↑↑
- Serum Parathyroid Hormone (PTH): ↑
- Serum $Ca^{2+}$: ↓ or Normal
- Serum $PO_4^{3-}$: ↓
X-Ray (Wrist AP view is best):
- Metaphyseal changes: Cupping, Fraying, Splaying
- Widening of the epiphyseal plate
- Generalized osteopenia & cortical thinning
- Looser's zones: Pseudofractures

⭐ High-Yield: The earliest biochemical marker to be elevated in rickets is Serum Alkaline Phosphatase (ALP).

Management - IAP Guidelines

Stoss Therapy (Oral/IM): Preferred method for rapid healing.
- Age < 1 yr: 3 doses of 100,000 IU at 4-week intervals.
- Age > 1 yr: 6 doses of 100,000 IU at 4-week intervals.
- Alternative (>1 yr): Single dose of 600,000 IU IM.
Calcium Supplementation: Crucial during initial therapy.
- Dose: 500 mg elemental calcium/day.
Maintenance Phase: After stoss therapy is complete.
- Dose: 400 IU/day + continued calcium.

⭐ Most skeletal deformities correct spontaneously with medical treatment. Orthopedic surgery is deferred until after rickets has healed and serum alkaline phosphatase is normal.

High-Yield Points - ⚡ Biggest Takeaways

Nutritional rickets, due to Vitamin D deficiency, is the most common metabolic bone disease in children.

Craniotabes (ping-pong skull) is the earliest sign.

Wrist & knee X-rays show metaphyseal cupping, fraying, and splaying.

Biochemical markers: ↓ Serum Phosphate, ↑ Alkaline Phosphatase (ALP), and ↑ Parathyroid Hormone (PTH).

Classic signs include rickety rosary, Harrison's sulcus, and limb deformities like genu varum.

Treatment often involves Stoss therapy (600,000 IU Vitamin D).

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