A 62-year-old man comes to the physician because of a swollen and painful right knee for the last 3 days. He has no history of joint disease. His vital signs are within normal limits. Examination shows erythema and swelling of the right knee, with limited range of motion due to pain. Arthrocentesis of the right knee joint yields 7 mL of cloudy fluid with a leukocyte count of 29,000/mm3 (97% segmented neutrophils). Compensated polarized light microscopy of the aspirate is shown. Which of the following is the most likely underlying mechanism of this patient's knee pain?

Calcium pyrophosphate deposition

Immune complex-mediated cartilage destruction

Bacterial infection of the joint

A 4-year-old boy is brought to the pediatrician by his mother for a routine medical examination. His medical history is relevant for delayed gross motor milestones. The mother is concerned about a growth delay because both of his brothers were twice his size at this age. Physical examination reveals a well-groomed and healthy boy with a prominent forehead and short stature, in addition to shortened upper and lower extremities with a normal vertebral column. The patient’s vitals reveal: temperature 36.5°C (97.6°F); pulse 60/min; and respiratory rate 17/min and a normal intelligence quotient (IQ). A mutation in which of the following genes is the most likely cause underlying the patient’s condition?

Fibroblast growth factor receptor 3

Runt-related transcription factor 2

Insulin-like growth factor 1 receptor

A 67-year-old male presents with left hip pain. Examination reveals mild effusions in both knees, with crepitus in both patellofemoral joints. He states his hearing has worsened recently and that he feels like his hats don't fit anymore. Bone scan reveals diffuse uptake in the calvarium, right proximal femur, and left ilium. Which of the following laboratory abnormalities would be expected in this patient?

Increased serum alkaline phosphatase

Decreased serum alkaline phosphatase

Increased serum parathyroid hormone

Decreased serum parathyroid hormone

A 23-year-old woman comes to the office because of a 2-day history of right knee pain. She says, "I can't run anymore because my knee hurts." The pain is localized "somewhere under the kneecap" and is achy, rated 5/10, but increases to 8/10 with prolonged sitting. She reports an occasional "popping" sound and sensation when she rises from a seated position. She has no history of trauma to the knee. She had a right clavicular fracture 2 years ago that was treated with a shoulder sling. She takes a daily multivitamin and has no known drug allergies. She does not smoke and drinks up to three glasses of wine weekly. Vital signs: Temperature 37°C (98.6°F), pulse 65/min, respirations 15/min, blood pressure 108/62 mm Hg. Height 173 cm (5 ft 8 in), weight 54 kg (119 lb), BMI 18 kg/m². Physical examination shows no acute distress. Pulmonary examination shows lungs clear to auscultation. Cardiac examination shows regular rate and rhythm with normal S1 and S2; no murmurs, rubs, or gallops. The abdomen is thin with no tenderness, guarding, masses, bruits, or hepatosplenomegaly. Extremities show no joint erythema, edema, or warmth; dorsalis pedis, radial, and femoral pulses are intact. Musculoskeletal examination shows diffuse tenderness to palpation over the right anterior knee, worse with full extension of the knee; no associated effusion or erythema; full, symmetric strength of quadriceps, hip abductors, and hip external rotators; crepitus with knee range of motion; and antalgic gait. Neurologic examination shows the patient is alert and oriented with cranial nerves grossly intact and no focal neurologic deficits. Which of the following is the most appropriate next step in management?

Intraarticular steroid injection

A 54-year-old female presents to her primary care physician with complaints of fatigue, constipation, and what the patient describes as "aching in her bones." Her medical history is significant for hypertension, well-controlled on lisinopril, and two prior kidney stones that both passed spontaneously without need for surgery. Vital signs are within normal limits, and physical exam is not significant for any notable findings. Preliminary lab work is ordered and reveals: calcium 11.6 mg/dL (normal range 8.5 - 10.9 mg/dL), phosphorus 2.1 mg/dL (normal range 2.4 - 4.1 mg/dL), and an elevated parathyroid hormone (PTH) level. Which of the following findings would most likely be expected on radiographic evaluation of this patient's hands?

Subperiosteal cortical thinning

Osteoid matrix accumulation around bony trabeculae

Joint space narrowing at the proximal and distal interphalangeal joints

Dense bone filling the medullary cavity of the phalanges and metacarpals

Ulnar deviation of the metacarpophalangeal joints

A 38-year-old woman was brought to the emergency department after she experienced dizziness and shortness of breath while walking with her friend. She recently immigrated to the United States and is unable to report her previous medical history. Physical exam reveals pallor underneath her eyelids. Labs are obtained with the following results: Hemoglobin: 8.4 g/dL Platelet count: 62,000/mm^3 Mean corpuscular volume: 89 µm^3 Reticulocyte count: 0.1% Lactate dehydrogenase: 175 U/L Which of the following is associated with the most likely cause of this patient's symptoms?

Administration of a 50S ribosomal inhibitor

Recent infection with a toxin producing gram-negative rod

A previously healthy 59-year-old man comes to the physician with a 6-month history of worsening headaches, difficulty chewing, and progressive hearing loss. Examination shows a mildly tender, 1-cm, hard swelling over the left maxilla. The remainder of the examination shows no abnormalities. Serum studies show a calcium concentration of 8.5 mg/dL, alkaline phosphatase activity of 112 U/L, and parathyroid hormone concentration of 310 pg/mL. Audiometry shows bilateral mixed conductive and sensorineural hearing loss. Which of the following processes is the most likely cause of this patient's condition?

Increased activity of nuclear factor-κB

Decreased activity of carbonic anhydrase II

Decreased expression of menin protein

Defective synthesis of type I collagen

Bone and joint pathology for USMLE Step 2 CK: High-Yield Pathology Lessons

Developmental & Genetic - Blueprint Bummers

Osteogenesis Imperfecta (OI): Brittle bones. AD defect in Type I collagen. Presents with fractures, blue sclerae, hearing loss. 📌 I = Type I collagen.
Achondroplasia: Most common non-lethal dwarfism. Gain-of-function mutation in FGFR3 gene → inhibits cartilage growth.
Osteopetrosis: "Marble bone disease." Defective osteoclast resorption → dense, brittle bones. Can cause pancytopenia, cranial nerve palsies.
Thanatophoric Dysplasia: Most common lethal dwarfism; severe FGFR3 mutation.

⭐ Osteogenesis imperfecta is associated with blue sclerae due to thin scleral collagen revealing underlying choroidal veins.

Metabolic Bone Disease - Brittle Bone Brigade

Osteoporosis: ↓ bone mass despite normal mineralization. T-score ≤ -2.5. Primary (post-menopause, senile) or secondary (e.g., steroids, hyperparathyroidism). Presents with vertebral compression fractures, kyphosis.
Osteomalacia & Rickets: Defective mineralization of osteoid (adults) or growth plates (children) due to ↓ Vitamin D. Leads to soft, weak bones, bone pain, and pseudofractures (Looser zones).
Paget Disease (Osteitis Deformans): Disordered bone remodeling. Phases: lytic (osteoclasts), mixed, sclerotic (osteoblasts). ↑ serum ALP. Mosaic pattern of woven/lamellar bone.

Paget disease of bone: Mosaic pattern of cement lines

⭐ High-output cardiac failure can occur in severe Paget disease due to the formation of arteriovenous shunts within the highly vascularized bone.

Bone Tumors - Skeletal Scare‑Fest

Benign Tumors
- Osteochondroma: Most common benign tumor. Bony stalk with a cartilage cap. Ages < 25.
- Giant Cell Tumor: Ages 20-40. Epiphysis of long bones (e.g., distal femur). "Soap bubble" on X-ray. Locally aggressive.
Malignant Tumors
- Osteosarcoma: Most common primary malignant bone tumor (excluding myeloma). Ages 10-20. Metaphysis of long bones. X-ray: Codman's triangle, sunburst pattern.
- Ewing Sarcoma: Ages < 15. Diaphysis of long bones. Histology: small, round, blue cells. X-ray: "onion skin" periosteal reaction. Associated with t(11;22) translocation.

Osteosarcoma X-ray: Codman triangle, sunburst pattern

⭐ Osteosarcoma risk is increased in patients with familial retinoblastoma, Li-Fraumeni syndrome, and Paget disease of bone.

Arthropathies - Joint Jeopardy

Osteoarthritis (OA): Degenerative "wear & tear." Asymmetric. Affects DIP, PIP, & weight-bearing joints. Non-inflammatory.
- Nodes: Heberden's (DIP), Bouchard's (PIP).
Rheumatoid Arthritis (RA): Autoimmune, inflammatory pannus. Symmetric. Affects MCP, PIP; spares DIP.
- Labs: ↑ Anti-CCP (specific), ↑ RF.
Gout: Monosodium urate crystals.
- Crystals: Needle-shaped, negatively birefringent (yellow when parallel to light).
Pseudogout (CPPD): Calcium pyrophosphate crystals.
- Crystals: Rhomboid, positively birefringent (blue when parallel).

⭐ Gout vs. Pseudogout: Remember "B-A-N-D" for birefringence: Blue when Add (parallel) for pseudo, Negative for gout, Decreased (yellow) when parallel.

High‑Yield Points - ⚡ Biggest Takeaways

Osteoporosis: Type 1 (postmenopausal) affects trabecular bone; Type 2 (senile) affects cortical bone.

Osteomyelitis: Most commonly caused by S. aureus; suspect Salmonella in sickle cell disease.

Giant cell tumor: Presents with a "soap bubble" appearance on X-ray in the epiphysis.

Ewing sarcoma: A small, round, blue cell tumor defined by the t(11;22) translocation.

Osteosarcoma: Shows Codman's triangle and a sunburst pattern in the metaphysis.

Gout: Characterized by needle-shaped, negatively birefringent monosodium urate crystals.

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