What is the cause of a 0.5 cm hard circular bone protrusion on the lateral side of the wrist in a 26-year-old female weightlifter with improving joint pain and a persistent vinegar smell around the area, despite normal X-ray results and no history of trauma or bleeding?

Differential Diagnosis: Lateral Wrist Bone Protrusion with Vinegar Odor

The most likely diagnosis is a ganglion cyst or carpal boss (bony prominence), with the vinegar smell representing a separate dermatologic issue unrelated to the underlying wrist pathology. The hard, fixed 0.5 cm protrusion on the lateral wrist in a young weightlifter with normal radiographs most commonly represents either a carpal boss or a small ganglion cyst that has become firm over time 1.

Primary Wrist Pathology Assessment

Most Likely Diagnoses for the Bony Protrusion

• Carpal boss: A bony prominence typically occurring at the base of the second or third metacarpal, presenting as a hard, non-mobile mass that can develop from repetitive stress in weightlifters 1
• Ganglion cyst: While typically soft and fluctuant, chronic ganglion cysts can become firm and may feel hard on palpation, especially when small 1
• Occult bone pathology: Though less likely with normal radiographs, small bone lesions may not be visible on plain films 1

Critical Red Flags to Exclude

You must actively exclude primary bone malignancy, particularly in this age group. Giant cell tumor of bone (GCTB) occurs most commonly between ages 20-40 and typically presents at the epiphyses of long bones near joints 1. While the wrist is not the most common location, the UK guidelines emphasize that:

• Persistent pain or a mass requires urgent MRI or referral to a bone sarcoma center if radiographs are normal 1
• A "normal" x-ray does not exclude primary malignant bone tumor 1
• Night pain is a red flag requiring further investigation 1

Since her pain is improving and she has no night pain, malignancy is less likely, but the persistent mass warrants advanced imaging.

Recommended Diagnostic Algorithm

Immediate Next Steps

1. Obtain MRI without IV contrast as the next imaging study 1

   • MRI is the most appropriate examination when radiographs are normal but a palpable mass persists 1
   • MRI can distinguish between ganglion cysts, carpal boss, occult fractures, and soft tissue masses 1
   • MRI demonstrates findings diagnostic for certain benign conditions (lipomas, hemangiomas, benign nerve sheath tumors) and can stage nonspecific masses 1

2. Ultrasound is an acceptable alternative for initial evaluation 1

   • US can accurately diagnose popliteal and ganglion cysts and determine vascularity of a mass 1
   • Given the superficial location of wrist structures, high-frequency (10-15 MHz) linear transducers provide excellent visualization 1
   • US allows dynamic examination and comparison with the contralateral side 1

If Imaging Shows a Ganglion Cyst

• Ultrasound-guided aspiration may provide both diagnosis and temporary relief for symptomatic cysts 1
• Most ganglion cysts in weightlifters can be managed conservatively with activity modification 1

If Imaging Shows a Carpal Boss

• This is a benign bony prominence that typically requires no treatment unless symptomatic 1
• Activity modification and avoiding direct pressure usually suffice 1

The Vinegar Smell: A Separate Issue

The vinegar odor is almost certainly unrelated to the bone protrusion and represents a dermatologic or metabolic phenomenon. This is critical to understand:

Possible Explanations for Vinegar Odor

• Bacterial colonization: Certain bacteria (particularly Corynebacterium species) produce propionic acid, creating a vinegar-like smell on the skin
• Eccrine bromhidrosis: Altered sweat composition from weightlifting/gym activities can produce acidic odors
• Fungal colonization: Though typically not described as "vinegar-like," some fungal infections can alter skin odor
• Metabolic causes: Rarely, certain metabolic conditions can alter body odor, but this would not be localized to one area

Important Clinical Caveat

There is no established medical literature linking bone pathology, ganglion cysts, or carpal bosses to vinegar odors. The temporal association is likely coincidental. The smell warrants:

• Thorough skin examination for subtle dermatitis, fungal infection, or bacterial colonization
• Trial of topical antibacterial wash (chlorhexidine or benzoyl peroxide)
• If persistent despite hygiene measures, consider dermatology referral for skin culture

Common Pitfalls to Avoid

1. Do not assume the smell indicates infection of the bone or joint - there is no discharge, erythema, warmth, or systemic symptoms to suggest septic arthritis or osteomyelitis 2

2. Do not delay advanced imaging - while radiographs are normal, persistent masses in young adults require MRI to exclude rare but serious pathology 1

3. Do not perform joint aspiration without clinical signs of infection - the hard, fixed nature of the mass and absence of inflammatory signs make septic arthritis extremely unlikely 2

4. Do not ignore the improving pain - this is reassuring and suggests against aggressive pathology, but does not eliminate the need to characterize the mass 1

Management Plan

Order MRI of the wrist without IV contrast to definitively characterize the bony protrusion 1. Simultaneously, address the vinegar smell as a separate dermatologic issue with improved hygiene measures and possible topical antibacterial treatment. If MRI shows a benign process (ganglion cyst or carpal boss) and the patient remains asymptomatic with improving pain, reassurance and activity modification are appropriate 1. However, if any concerning features emerge on MRI, urgent referral to orthopedic surgery or a bone sarcoma center is warranted 1.