What is the most likely diagnosis for a 25-year-old male with a 1.5 cm circumscribed mass in his right testis, showing uniform nests of cells with distinct cell borders and round nuclei with prominent nucleoli on microscopic examination?

Diagnosis: Seminoma

The most likely diagnosis is seminoma (option b), based on the characteristic histologic findings of uniform nests of cells with distinct cell borders, round nuclei, and prominent nucleoli in a young male with a circumscribed testicular mass. 1

Histopathologic Features Supporting Seminoma

The microscopic description provided is pathognomonic for seminoma:

• Uniform nests of cells with distinct cell borders - This describes the classic "sheets" or nests of seminoma cells separated by fibrous septa containing lymphocytic infiltrate 1
• Round nuclei with prominent nucleoli - These are the hallmark cytologic features of seminoma cells, which are large cells with clear cytoplasm, round nuclei, and conspicuous nucleoli 2
• Circumscribed mass - Seminomas typically present as well-defined, homogeneous masses 2

Why Other Options Are Incorrect

• Teratoma (option a): Would show heterogeneous tissue elements from multiple germ layers (ectoderm, mesoderm, endoderm) with mature or immature tissues, not uniform cells 1

• Embryonal carcinoma (option c): Demonstrates pleomorphic cells with indistinct borders, overlapping nuclei, and prominent nucleoli arranged in solid sheets, glands, or papillary structures - distinctly different from the uniform appearance described 1

• Choriocarcinoma (option d): Shows a biphasic pattern with cytotrophoblasts and syncytiotrophoblasts, hemorrhage, and necrosis - not uniform nests of cells 1

Clinical Context Supporting Seminoma

• Age 25 years: Seminoma is the most common testicular malignancy in men aged 15-40 years, with peak incidence in the third decade 1, 2
• Small, circumscribed mass (1.5 cm): Consistent with early-stage seminoma presentation 1
• Painless testicular enlargement: Classic presentation for seminoma 1

Critical Next Steps

Immediate management requires: 1

• Serum tumor markers (AFP, β-hCG, LDH) before any surgical intervention
• Normal AFP is mandatory for seminoma diagnosis - any elevation indicates nonseminomatous component and changes management entirely 1
• β-hCG and LDH may be elevated in pure seminoma 1
• Radical inguinal orchiectomy (not scrotal approach) 1
• Sperm banking discussion before orchiectomy 1
• CT chest/abdomen/pelvis for staging after orchiectomy 1

Common Pitfall

Never assume pure seminoma without confirming normal AFP levels - if AFP is elevated, the tumor must be managed as nonseminomatous germ cell tumor regardless of histologic appearance, as even microscopic nonseminomatous elements mandate different treatment 1