Tophi, which are deposits of monosodium urate crystals, are a hallmark of chronic tophaceous gout. They are most commonly found in certain characteristic locations. The question asks for the region where tophi are not found among the options given: prepatellar bursae, muscle, helix of ear, and synovial membrane.

Analysis of Each Option:

1. Prepatellar bursae:
   Tophi are well-documented within and around bursae, including the prepatellar bursa, which is a frequent site due to its superficial location and proximity to joints prone to trauma or chronic irritation. Clinical reports and imaging studies have demonstrated the presence of tophi in various bursae including the prepatellar bursa(Akn, 2015), as well as the elbow (olecranon bursae) and others(Garcia, 2023).

2. Muscle:
   Tophi deposition within muscle tissue is extremely uncommon and is not described as a typical or even rare manifestation of gout in the human literature reviewed here. Gout predominantly targets areas with low blood flow (cooler temperatures) or locations with previous microtrauma, such as periarticular tissues, tendons, bursae, and subcutaneous tissue(Fodor, 2014). Although some case reports in the literature have described soft tissue and subcutaneous involvement, tophi are not commonly, if ever, identified within muscle fibers themselves(Zahoor, 2022). This makes muscle the best candidate for the "except" in the question.

3. Helix of ear:
   The helix of the ear is a classic extra-articular site of tophus formation, likely due to its relatively low temperature and reduced blood supply. Numerous clinical studies and case series have documented auricular tophi(Joshi, 2025), and pathological investigations have confirmed urate deposits in this site(Cusack, 2019).

4. Synovial membrane:
   Tophi frequently develop within or adjacent to synovial membranes, particularly in chronic gout. Synovial and periarticular tissues are prime targets, and sonographic as well as pathological studies confirm this localization(Fernandes, 2010), (Sun, 2019).

Summary Table:

Correct Answer:
b. Muscle

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