A 26-MONTH-OLD GIRL was evaluated for stunted growth (weight, height, and head circumference less than the 5th percentile for age) and a 2-month history of altered bowel patterns with perianal pain. The decelerating growth pattern was first noted between the ages of 6 and 12 months, when her weight and length for age-growth percentiles fell from the 25th and 50th, to 5th and 25th, respectively. They later fell to less than the 5th percentile at the time of consultation. The 3-day diet record obtained at consultation revealed an average caloric intake of 121 kcal/kg per day (protein, 12%; carbohydrate, 57%; fat, 31%). Loose stools alternating with constipation characterized the altered bowel pattern. There was no history of mucus in stools, melena, or hematochezia. The perianal pain was most prominent when she sat in her car seat and immediately prior to bowel movements. She was born after a full-term pregnancy and had normal gross motor milestones, fine motor skills, and language development. Apart from small stature and minimal subcutaneous fat tissue, the rest of her physical examination results were within normal ranges. There was no anal fissure or other external perianal lesion. Laboratory tests included a normal hemogram, hepatic panel, sweat chloride, serum vitamin E level, and antigliadin/antiendomysial antibodies negative for celiac disease. Upper gastrointestinal endoscopy showed normal small-bowel mucosa and histology. The digital rectal examination performed prior to flexible sigmoidoscopy (normal) revealed a firm presacral mass that was further delineated by axial computed tomography (Figure 1). The serum α-fetoprotein level subsequently obtained was 1980 ng/mL (normal range, 0.6-11.1 ng). Histologic sections from the tumor mass resected at laparotomy are seen in Figure 2.