What is the next investigation for a male patient presenting with a well-demarcated, red, painful lesion on the thigh?

Clinical Assessment of a Well-Demarcated, Red, Painful Thigh Lesion

The next investigation should be a plain X-ray (Option C) of the thigh in two planes, as this is the mandatory first-line imaging for any suspected bone or deep tissue pathology presenting with these symptoms. 1

Diagnostic Approach

Why X-ray First?

The clinical presentation of a well-demarcated, red, painful lesion on the thigh raises concern for potential underlying bone involvement, soft tissue pathology, or infection that could extend to deeper structures. Conventional radiographs in two planes should always be the first investigation for any suspected bone or deep tissue lesion. 1

• Plain radiographs can identify bone destruction, periosteal reaction, soft tissue calcification, or gas formation that would indicate osteomyelitis, bone tumor, or necrotizing infection 1
• X-rays are rapid, inexpensive, and provide critical information about whether deeper structures are involved 1
• The presence of pain (especially if persistent or night pain) warrants radiological assessment to exclude bone pathology 1

Why Not the Other Options?

Punch biopsy (Option A) would be premature without first ruling out underlying bone or deep soft tissue involvement:

• Performing a biopsy before adequate imaging can contaminate tissue planes and compromise future surgical management if malignancy is present 1
• The biopsy tract itself can spread infection or tumor cells 1
• Staging studies should be obtained before biopsy to guide the location and approach of tissue sampling 1

MRI (Option B) is not the first-line investigation:

• MRI is indicated only when malignancy cannot be excluded with certainty on plain radiographs 1
• It is the next step after X-ray if bone sarcoma or deep soft tissue pathology is suspected 1
• Ordering MRI first is unnecessarily expensive and time-consuming when X-ray may provide sufficient information 1

Ultrasound (Option D) has limited utility as the primary investigation:

• While ultrasound can assess superficial soft tissue changes and guide needle procedures, it cannot adequately evaluate bone involvement 2
• It is more useful as an adjunct for guiding biopsies after the diagnosis is narrowed 1

Clinical Pitfalls to Avoid

• Never assume a superficial presentation means superficial disease: A red, painful lesion could represent overlying cellulitis from underlying osteomyelitis or a bone tumor breaking through the cortex 1
• Do not biopsy before imaging: This fundamental principle prevents tissue contamination and allows proper surgical planning if malignancy is discovered 1
• Consider age-appropriate differential diagnoses: In younger patients, bone sarcomas are more likely; in older patients, consider metastases, myeloma, or infection 1

Next Steps After X-ray

If the X-ray shows:

• Normal findings with persistent symptoms: Proceed to MRI to evaluate soft tissue and bone marrow 1
• Suspicious bone lesion: MRI of the whole bone with adjacent joints for staging, followed by referral to a specialized center before biopsy 1
• Signs of infection: Consider additional imaging and infectious workup as clinically indicated 1