The correct choice is (2) B and C.
The early exaggerated loss and its recovery over a week are best explained by transient circulatory impairment around the lesion and its later recovery, not by true axonal regeneration.

Interpreting the scenario

• Electrolytic lesion of the caudate nucleus initially caused increased muscle tone.

• Within 7 days, tone decreased toward normal.

• Electrolytic lesions damage not only target neurons but also surrounding tissue (edema, ischemia), which can produce a temporary functional deficit larger than the permanent lesion.

• As edema resolves and local perfusion improves over days, some function returns even though destroyed neurons do not regenerate.

Evaluating each explanation

A. “Functional recovery was due to plastic changes of nervous system.” – Not best here

• Neural plasticity (synaptic reorganization, sprouting) can contribute to long‑term recovery, but it usually acts over weeks to months, not just a few days.

• The time-course and classic physiology of acute lesions point more to reversal of transient dysfunction than to plastic rewiring.

B. “Surrounding brain tissue was non‑functional due to circulatory insufficiency immediately after surgery, causing greater functional loss.” – Correct

• Electrolytic lesion produces local edema, microvascular spasm, and bleeding, which can depress function in tissue bordering the lesion.

• This “shock” or diaschisis makes the initial deficit larger than what the permanent lesion alone would cause.

• So B is a valid explanation.

C. “Circulatory status in surrounding tissue recovered with time, resulting in partial functional recovery.” – Correct

• As swelling and vascular compromise subside over several days, perfusion improves and reversibly impaired neurons recover, reducing muscle tone toward normal.

• This matches the 7‑day improvement described.

D. “Degenerating nerve fibers were regenerated, underlying functional recovery.” – Incorrect

• In the adult central nervous system, especially with electrolytic lesions, true axonal regeneration and reconstitution of the original circuitry is extremely limited.

• Recovery within a week is too rapid to be due to structural regeneration of long tracts; rather it reflects recovery of stunned but viable neurons.

Why option (2) is correct

• The pattern of early severe deficit followed by partial recovery in a few days is best accounted for by:

  • B: transient functional loss in peri‑lesional tissue due to circulatory disturbance.

  • C: subsequent restoration of blood flow and function.

• Explanations A and D rely on plasticity or regeneration, which do not fit the short timeline or known CNS biology as well.

Therefore, the correct combination is B and C (option 2).