diagnose this?

Pellagra.

The Casal collar or Casal necklace is a clinical sign in which there is erythematous pigmented skin rash in the distribution of a broad collar (dermatomes C3 and C4). It is seen in patients with pellagra

Pellagra is a systemic nutritional wasting disease caused by a deficiency of vitamin B3 (niacin), which is an essential component of several coenzymes.

Niacin is essential for adequate cellular function because of its required roles in 2 similar but distinct coenzymes (ie, nicotinamide adenine dinucleotide [NAD] and nicotinamide adenine dinucleotide phosphate [NADP]). Both of these are cofactors that can be recycled by serving as both oxidizing (NAD, NADP) and reducing (NADH, NADPH) agents.

Pellagra defines systemic disease as resulting from a marked cellular deficiency of niacin. It is characterized by 4 "D's": diarrhea, dermatitis, dementia, and death. Diagnosis of pellagra is difficult in the absence of the skin lesions, and is often facilitated by the presence of characteristic ones. The dermatitis begins as an erythema. Acute pellagra resembles sunburn in its first stages, but tanning occurs more slowly than typically in sunburn.

Nicotinamide or niacin taken orally is usually effective in reversing the clinical manifestations of pellagra. Because patients are often malnourished and suffer from other vitamin deficiencies, provision of a high-protein diet and B-complex vitamins is needed for complete restoration of health.