A patient with a well-fixed cobalt-chromium primary total knee arthroplasty develops persistent, massive swelling, disabling pain, and a widespread, highly pruritic eczematous skin rash without any serological (CRP/ESR) or synovial fluid evidence of infection. Which of the following is the most appropriate next diagnostic step to evaluate for a suspected metal hypersensitivity reaction?

Explanation:
The correct option is b. Metal hypersensitivity (particularly to nickel, cobalt, and chromium) is a rare but well-documented type IV delayed hypersensitivity immune response that can cause severe pain, effusion, and systemic cutaneous manifestations in the presence of a well-fixed implant. Because it mimics infection, infection must first be rigorously ruled out. Once ruled out, diagnosing metal allergy relies on specialized immunological testing. The Lymphocyte Transformation Test (LTT) is a highly specific blood test measuring the proliferative response of the patient's T-cells when exposed to metal ions. Cutaneous patch testing is also utilized, though it is slightly less specific for deep intra-articular reactions. Reference: Thomas P et al. Hypersensitivity to implant materials. Orthopedics.