Nephrolithiasis (Kidney Stone): The presence of a 0.6 cm echogenic calculus in the renal pelvis is highly suggestive of a kidney stone. The location and echogenic nature of the finding are classic for this diagnosis.

Blood Clot: Although less likely than a stone, a blood clot in the renal pelvis could appear echogenic and might be considered, especially if there's a history of trauma, infection, or other conditions predisposing to bleeding.
Papillary Necrosis: This condition could potentially present with an echogenic lesion in the renal pelvis, especially if there's a history of diabetes, analgesic abuse, or other predisposing factors.

Urothelial Carcinoma (Transitional Cell Carcinoma): Although less common, a tumor in the renal pelvis could present as an echogenic mass. Given the potential for malignancy, it's crucial not to miss this diagnosis, even if it's less likely.
Xanthogranulomatous Pyelonephritis: This rare form of chronic pyelonephritis could mimic a stone or mass on imaging and is important to consider due to its potential for significant morbidity if not treated appropriately.

Malacoplakia: A rare inflammatory condition that could potentially present with an echogenic lesion in the renal pelvis, although it's much less common than other diagnoses.
Fungal Ball: In immunocompromised patients, a fungal infection could lead to the formation of a fungal ball in the renal pelvis, which might appear echogenic on ultrasound.