An 18-month-old boy is brought to the doctor’s office for evaluation of abdominal pain. The boy looks emaciated and he is now significantly below his growth chart predicted weight. The family history is non-contributory. The vital signs are unremarkable. On physical examination, a non-tender mass is felt in the upper part of the abdomen. A magnetic resonance image (MRI) scan of his abdomen demonstrates a mass in his right adrenal gland. Biopsy of the mass demonstrates an abundance of small round blue cells. With this biopsy result, which 1 of the following findings would confirm the diagnosis?

Elevation of vanillylmandelic acid in the urine

MRI showing the intrarenal origin of the mass

Radiograph of the bone showing the presence of lytic bone lesion with periosteal reaction

A 1-month-old boy is brought to the physician because of a 5-day history of generalized fatigue and multiple episodes of vomiting which is most pronounced after formula feeding. His vomiting progressed from 2–3 episodes on the first day to 6–8 episodes at present. The vomitus is whitish in color. The mother reports that he has been very hungry after each episode of vomiting. The patient was born at 38 weeks' gestation and weighed 3100 g (6 lb 13 oz); he currently weighs 3500 g (7 lb 11 oz). He appears irritable. His temperature is 37.1°C (98.8°F), pulse is 130/min, respirations are 43/min, and blood pressure is 74/36 mm Hg. Examination shows dry mucous membranes. The abdomen is soft and not distended. There is a round mass palpable in the epigastric region. The liver is palpated 1 cm below the right costal margin. Laboratory studies show: Hemoglobin 15.3 g/dL Leukocyte count 6300/mm3 Platelet count 230,000/mm3 Serum Na+ 133 mEq/L K+ 3.4 mEq/L Cl- 92 mEq/L Glucose 77 mg/dL Creatinine 1.0 mg/dL A urinalysis shows a decreased pH. Which of the following is the most appropriate next step in the management of this patient?

Administer IV 0.9% NaCl and replace electrolytes

Perform emergency pyloromyotomy

Obtain CT scan of the abdomen with contrast

Rhabdomyosarcoma for USMLE Step 2 CK: High-Yield Pediatrics Lessons

Overview - Malignant Muscle Mayhem

Most common soft tissue sarcoma in children, arising from primitive mesenchymal cells (rhabdomyoblasts).
Bimodal age distribution: peaks at 2-6 years and 15-19 years.
Common Sites:
- Head & Neck (esp. orbit, parameningeal)
- Genitourinary (bladder, prostate, vagina)
- Extremities

⭐ Sarcoma botryoides, an embryonal subtype, presents as a submucosal "bunch of grapes" (botryoid) mass, classically in the vagina or bladder.

Sarcoma botryoides gross specimen with grape-like masses

Histopathology - The Cellular Suspects

Rhabdomyoblast: The diagnostic cell. Can be round, or elongated (tadpole/strap cells) with eosinophilic, granular cytoplasm.
- Cross-striations, resembling skeletal muscle, are pathognomonic but rarely seen.
Major Subtypes:
- Embryonal (ERMS): Most common (~60%). Spindle cells in a myxoid stroma. Includes the Sarcoma Botryoides variant ("bunch of grapes").
- Alveolar (ARMS): Aggressive. Uniform, round cells lining fibrous septa, resembling lung alveoli.
Immunohistochemistry (IHC) Markers: Myogenin (most specific), Desmin, and MyoD1.

⭐ Alveolar rhabdomyosarcoma is characterized by chromosomal translocations, typically t(2;13), creating a PAX3-FOXO1 fusion gene, which is linked to a poorer prognosis.

Clinical Features - Symptom Hotspots

Site-dependent presentation. Mass effect is the primary driver of symptoms.
Head & Neck (Most Common: ~40%)
- Orbit: Proptosis (most common orbital malignancy in childhood).
- Parameningeal (Nose, Sinuses, Middle Ear): Nasal/aural discharge, obstruction, cranial nerve palsies.
Genitourinary (~25%)
- Bladder/Prostate: Hematuria, urinary obstruction, pelvic mass.
- Vagina/Uterus: 💡 Sarcoma botryoides - polypoid, grape-like mass protruding from the vagina.
- Paratesticular: Painless scrotal mass.
Extremities (~20%): Rapidly enlarging, firm, painless mass.

⭐ Sarcoma botryoides is the embryonal subtype of rhabdomyosarcoma, classically presenting as a "bunch of grapes" mass in the vagina or bladder of young girls.

Diagnosis & Staging - The Workup Blueprint

Biopsy is Gold Standard: Incisional or excisional biopsy is mandatory for diagnosis.
- HPE: Small, round, blue cells; may show tadpole or strap cells.
- IHC Panel: Desmin, Myogenin, and MyoD1 are key markers.
Staging Workup: A systematic search for local and distant disease.

⭐ The most critical factor for staging is the post-surgical Clinical Group (IRS), which assesses residual disease after the initial surgery.

Embryonal Rhabdomyosarcoma Histopathology with Strap Cells

Management - The Treatment Takedown

Multimodal approach is key: Surgery + Chemotherapy ± Radiotherapy.
Goal of Surgery: Wide local excision with negative margins, if feasible.
Chemotherapy: Backbone of treatment. Standard regimen is VAC (Vincristine, Actinomycin D, Cyclophosphamide) or IVA (Ifosfamide, Vincristine, Actinomycin D).

⭐ Treatment is stratified by the Intergroup Rhabdomyosarcoma Study (IRS) post-surgical clinical group, which assesses the extent of residual disease after surgery.

High‑Yield Points - ⚡ Biggest Takeaways

Rhabdomyosarcoma is the most common soft tissue sarcoma in childhood, with a bimodal peak at 2-6 years and adolescence.

The most common site is the head and neck (especially the orbit), followed by the genitourinary tract.

Embryonal subtype is most frequent, has a better prognosis, and includes the sarcoma botryoides (grape-like) variant.

Alveolar subtype has a worse prognosis, affects extremities, and is linked to t(2;13) translocation.

Key diagnostic markers include Desmin and Myogenin.

The characteristic cell is the rhabdomyoblast (tadpole or strap cell).

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Rhabdomyosarcoma