Atypical Pruriginous Pustular Eruption Preceding Locally Advanced Rectal Cancer: A Case Report and Gut–Skin–Tumour Axis Hypothesis

Background and Clinical Significance: Cutaneous paraneoplastic phenomena are infrequently characterised in colorectal cancer (CRC), and chronic pruriginous inflammatory eruptions in particular have received limited attention. In older adults, persistent treatment-resistant dermatoses of unclear aetiology may represent overlooked extraintestinal diagnostic clues to occult malignancy, including potentially curable CRC. Faecal immunochemical testing (FIT) for occult bleeding is a low-cost, non-invasive tool whose role outside conventional alarm-symptom triage remains underexplored. Case presentation: A 72-year-old woman presented for outpatient evaluation with several months of pruriginous, pustular, and crusted symmetric eruption involving the dorsal aspects of the limbs, refractory to standard dermatologic treatment, and without gastrointestinal symptoms. A non-invasive systemic stool-based work-up demonstrated detectable faecal haemoglobin (iFOBT), mildly elevated faecal calprotectin (51.6 mg/kg, ULN 50 mg/kg), markedly elevated faecal alpha-1-antitrypsin (631 µg/mL; 2.3× ULN), and predominance of Escherichia coli on stool culture. Colonoscopy revealed a locally advanced rectal adenocarcinoma; staging classified the lesion as cT3N1M0. The patient received long-course neoadjuvant chemoradiotherapy (50 Gy, concurrent capecitabine) followed by low anterior resection with total mesorectal excision and pathological complete response (ypT0N0, R0), and adjuvant capecitabine. The cutaneous eruption resolved progressively in parallel with antineoplastic therapy without specific dermatologic intervention. The patient remains in remission at over 36 months. Conclusions: Persistent, unexplained, treatment-resistant pruriginous/pustular cutaneous eruptions may, in selected patients, coincide with an underlying malignancy, including colorectal cancer, and should prompt careful individualised clinical assessment, including review of age-appropriate colorectal cancer screening status. This single case raises the hypothesis that quantitative faecal immunochemical testing (FIT) may be prospectively evaluated as a low-cost, non-invasive triage tool in carefully selected patients aged ≥50 years with persistent dermatoses of unclear aetiology, even in the absence of gastrointestinal symptoms. Positive FIT results should be managed according to established local colorectal referral pathways. NICE diagnostics guidance DG56 supports FIT use in symptomatic adults with suspected lower gastrointestinal pathology; however, any extension of FIT to extraintestinal presentations remains investigational and requires formal validation through prospective studies assessing diagnostic yield, cost-effectiveness, and stage distribution.